It is always best to embed an image from it's source rather than download and re-upload or attach a *copy* of it to your work. Why? Because the original is always best - and it helps give credit where credit is due - your source.
For Google Sites
For Docs and Presentations
NOTE: It is very important that the image owner or collection owner has granted permissions for their image(s) to be reused.
NOTE: if the image does not display or there are other problems, the image owner or the photo sharing site may not allow re-use of their images via URL link. You may elect try an alternate method if you know the owner allows re-use.