You can embed an album from Picasa. You can add a URL from any Picasa account, it does not need to be associated with your school Apps account. In fact, you can include an album from someone else entirely provided they have given you or the public access to the photo/album. 

Please note: When you are using Google Sites on your school Apps account, you will not have the ability to add a single photo as your Apps account is not associated with a Picasa account. You will be able to paste in any URL of a Picasa web album.

To insert a Picasa web album on your page, follow these steps:

1. Make sure you are in editing mode on your page (if you see the Save or Cancel buttons, you are in editing mode, otherwise, click the Edit page button).
2. Click Insert and select Picasa Web slideshow.
3. Paste the URL of the album you'd like to insert.
4. Select the size of the slideshow and how you want to display it on the page.
• Slideshow size: There are several options for the size of your slideshow. You can always change this setting if after you see the album on the page you feel it is too small or too large. Each option specifies the exact pixel size for small, medium, large, extra large, and extra extra large.
• Show captions: This option allows you to include any captions of the photos in the slideshow.
• Autoplay: By default, this option is checked. Autoplay means that when any visitor that comes to the page, the slideshow will begin immediately. If you wish to give the visitor more control over starting the slideshow, decheck this option. 
• Gadget display: These settings apply to the border and heading of the gadget.
5. Click Save.
6. The Picasa album gadget will appear as a blank 'box' on your page. Click Save again at the top of the page to save all your changes and view the slideshow gadget.