Insert a Photo Album Slideshow

In addition to inserting as many images as you wish to your google site, you can also insert a slideshow - a collection of images that will rotate automatically through selected images and with viewer controls to pause, move forward, and backwards in the slideshow.

Now that Google's Photos has broken out of Google + as a separate tool, This page is now under reconstruction.

In short, you will need to create a "collection" in Google Photo and "share" this collection (under the More Options 3 dots).

Then, once in your Google site page, go to the Insert menu > Google+ (yah, no Photos yet) > Photo Album.

Hope to have more descriptive process soon.