To share a picture in a forum, blog,... often you need a direct link to the jpg so it would be shown embedded in a post.
Using the following steps you can get a direct link... but mind, this procedure isn't documented by Google, so even though it is not likely, the behavior could change in the future.
Go to the image you want a direct link to on photos.google.com
Right-click the image and choose:
in Chrome: "Copy image URL"
in Internet Explorer: "Copy"
in Firefox: "Copy image location"
Now you can use "Paste" to put the link wherever you like. This is an example of such a link:
(Optional) If you would like to have a shorter link to use, you can always use any url shortener on it, eg. https://goo.gl/, https://bitly.com/,...
(Optional) Some sites want to have an image extension at the end of the URL, you can try adding either -tmp.jpg or ?.jpg at the end:
If you want to get an image of a specific size, you can edit the link above as explained here.
There are different parameters you can use to get different effects, but none are documented by Google, so the behavior could change or disappear at any moment.
You can always change the number(s) used in the parameters explained below with any other number to get a different size. If you request a size larger than the "physical" size uploaded to Picasa Web Albums or Google Photos, the physical size of the image will be returned.
These are the "known" parameters by the Picasa TCs... If you know of any other, please post your findings on https://support.google.com/photos
"s400": this gets the image with width or height (whichever is larger in the original image) resampled to 400 pixels. The other dimension will be 400 as well in case of a square images, otherwise it will be proportionally smaller.
landscape image (original: 2048 x 854 pixels, result: 400 x 167 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=s400
portrait image (original : 1196 x 2048 pixels, result 234 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=s400
"w400": this gets the image resampled so the width is always 400 pixels.
landscape image (original: 2048 x 854 pixels, result: 400 x 167 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=w400
portrait image (original : 1196 x 2048 pixels, result 400 x 685 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=w400
"h400": this gets the image resampled so the height is always 400 pixels.
landscape image (original: 2048 x 854 pixels, result: 960 x 400 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=h400
portrait image (original : 1196 x 2048 pixels, result 234 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=h400
"w800-h400": this gets the image resampled in such a way so the width will be maximum 800 pixels and the height will be maximum 400 pixels. So one of the dimensions will be the exact size specified, the other dimension will be the size specified or smaller.
landscape image (original: 2048 x 854 pixels, result: 800 x 334 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=w800-h400
portrait image (original : 1196 x 2048 pixels, result 234 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=w800-h400
"...-c", eg: "w800-h400-c": this gets the image so it is exactly this size. The source image is first resampled so the image is fitting for the limiting dimension, then the other dimension is cropped so the image is the exact size asked. When cropping, it is just the centre of the picture that is kept.
landscape image (original: 2048 x 854 pixels, result: 800 x 400 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=w800-h400-c
portrait image (original : 1196 x 2048 pixels, result 800 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=w800-h400-c
"...-p", eg: "w800-h400-p": this gets the image so it is exactly this size. The source image is first resampled so the image is fitting for the limiting dimension, then the other dimension is cropped so the image is the exact size asked. When cropping, some algorithm searches the most interesting part of the image, so the result of the cropping doesn't always keep the center of the picture.
landscape image (original: 2048 x 854 pixels, result: 800 x 400 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=w800-h400-p
portrait image (original : 1196 x 2048 pixels, result 800 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=w800-h400-p
"s0", "w0", "h0", "w0-h0": this will get the image in it's original size, without any resampling nor cropping.
landscape image (original: 2048 x 854 pixels, result: 800 x 400 pixels):
https://lh3.googleusercontent.com/sZl-o9TGNYXucrPNHesxaMLXajhcPYqw43zojzHJ-y1yYYeQaNtJMrPUQImKgak3paKOMbEm0Av0e5bKG8_z31m1xVGN8J3x-EIAfgeETIhrLxwsw7xWEGstIuwyKYuHjOLFPCcvqIBY944PWFHBhgSEsVERXJljVEwPTD1xpJHhi5gHieiZcjl-rJ734bFiDxG1GzTxkX5nRc9lkRhtNHMdHDOSswMU-dgp8itMF8lTdEztOYz_bE_8H2FxN5NtCBmeOvxTi7f31wM2zrAE7oOzzeVy1_hYLFAWuXJ0CNqDfc-J-Ui9HY1RILj9Z1nYvEKGGDSTiT3tzysWHO9Vn6rXDFKE9TUGNE1_Z9_EaQ7B_HIU_z7oq2Hmmikl1Ap5t7N_pEI44ZhUGbIVirHKJyvc2LgtJgczCeNEgH7SnvvYBCM-OMWX5MnB949rXYn678iZyz7Q16wjPMevr1IQU4zfdOE2xoJQi8x3FhpD2-3moUoilZRBR5OQi-KII4hRYrEaRFnAiGqYeFz3Rqwx_Yw97kROwh2AnfhT03700AHkFqmYobTXjq0Q3IQpjcYQZ3vbtw=s0
portrait image (original : 1196 x 2048 pixels, result 800 x 400 pixels): https://lh3.googleusercontent.com/9XANu0kkl94p8G9WgLUL14Ot1ROi1a1Hbw1Tpzb78Z9HaMsxywAA0sbLm0cwWt15PIfIHxUmds_7QONbYkowaWAMXbBSKFCb6SwfyKOWRV4FfHqJOXdqCVvwEPfEJiIRRmCf5frq8W8YcLbi9fj5dvFJezbcTpgqz-OKcWh9MaI0Pj6IaKgrQPkTq6dgjSQIpkqhdbJYWPMLvWrHxiyPWDZEmMGRIzd8cidTCVQHG3uPk8MowwnBpgy2Ptr3U_PU8JtuN3u90Xnp-noeGSYOMOmgRWBFig1rokY9AQjAPp6-e40OTGSjdpmJhomrVDyLZIBtPvOwS9_5TlNmtLG2x5i23rBQmuVyqSAJsshervLggFikmPGHx4VxfaRbiih2pH47F6mvjbY5U14yKCGFEO8PqdYmBpAS2hWjme5evNuN8Uj6fbvXMAAQ1KCOVfPHEtjz0LPJrntsGj_cy0RiVvN2YV6CMe2c0YPadHRDbKUbwQdod99HB6q6nSzzm8qWXmpaY8QSe9A0wdm7Uf5mNE8vNNGMLIg4UBgxZNfrGeom_Rvubt2SOufPsV8-RT7ZCsumOg=s0
"...-no", eg: "w800-h400-no": this gives a slightly bigger image as a result (for the image I looked into), haven't found out yet what the exact impact is.