While you can use the "Insert" > "Image" option this will quickly use up the 100MB allowed for each Google site, so a better way is to use Google Drive.
Use the free google drive permalink service to create a permalink to your image:
Return to google sites
From the Google search home page, you will see the "Drive" option at the top.
In the Public or Private folder (depending on whether you are happy with everyone seeing your image (public) or if you want to protect it (private) )
Google presentation (Googles version of powerpoint) is an easy way to create an image slideshow. Open Google Drive, navigate to your portfolio folder, click on Create and choose presentation from the drop down.
Add images by clicking on Insert >> Image and choosing either an existing image in your google drive, URL, capture from webcam or uploading one from you computer. Add new slides and images as required. Set any transitions, add text or other graphics. Name your presentation.
Return to your portfolio. Click on Insert >> Presentation. Alter size, name and other parameters as required. Save.
From the Google search home page, click on your name at the far left:
Hover your mouse over the photos icon and click +ADD PHOTOS
Following the onscreen prompts, name your album, drag or upload photos and click Create Album. When prompted to share your album, click CANCEL.
To make this album public so viewers of your portfolio can see the photo, click on Sharing Settings.
Locate your album and change the viewing permissions to public and click done.
Create as many albums as you like with different permissions depending on your requirements.
Ensure editing is on. Click on Insert >> Picasa Photo and follow the onscreen instructions.
Turn editing on. Click Insert >> Picasa web slideshow and select the album you wish to display.
Free storage limits
Photos up to 2048 x 2048 pixels and videos up to 15 minutes won't count towards your free storage.
Automatic resizing
All photos uploaded in Google+ will be automatically resized to 2048 pixels (on their longest edge) and won't count towards your free storage quota (method above).
All photos uploaded from the Picasa software or in Picasa Web Albums over the free size limit will count towards your 1 GB of free storage. When you reach your storage limit, any new photos you upload larger than the free size limit will be automatically resized to 2048 pixels (on their longest edge).