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.
Here are some general instructions on the process, followed by a short video demonstrating these steps.
1. The first thing you need to do is upload some images to Google Photos (Picasa Web Albums). Go to the More menu and select Photos. This will take you to a free tool by Google to store and display your photos publicly or privately. (Note... this may take you to Google+ Photos).
You can also download and install Picasa, Google's Photo editing, organization, and more software (great for PC users). This software allow you to create albums on the web and will auto update your slideshow.
2. Once you have created a New Album and upload the images you wish to use, there are lots of fun tools in Picasa Web Albums to edit, organize, and add captions to your images.
3. Important... must set your web album to "public" so the slideshow can be viewed from your Google Site. In Picasa Web Album, go to the Edit tab and select Album Properties. A little ways down the page you will find a Share menu - select Public.
4. Now, before you leave Picasa Web Albums, go to the left hand side of the screen (while in your album) and find and click on "Link to this Album". This will give you the URL you will need when inserting it into your Google Site. So go ahead and copy the URL (not the HTML code). That's it. Now its time to return to your Google Site.
5. Once back into your Google Site, select or create the page you wish to place the slideshow.
Click on the Edit button of the page.
Place in cursor in the location you wish to place the presentation.
Go to the Insert menu > select Picasa Web Slideshow.
6. A window will appear to allow you pick the album in Picasa Web you would like to insert. It will also allow you to select some basic setting, mostly to set the size of the slideshow - pick the size that suits your needs.
7. Once posted to your web page, the slideshow will NOT display until you SAVE the page. Further adjustment can be made to the slideshow (in the page Edit mode) by clicking once and tools will appear (just like images).
Here is a short video that will review the Insert a slideshow process using PicasaWeb Album (in case you still have access).
Here is a short video that will review the Insert a slideshow process using Google+.