While helping one of our teachers I found she used this cool hack.
I love how Google make this stuff so easy.
Add an animated header to your google site (in three easy steps)
1. Go to Google Photos and select the photos you want in your animated header
2. Once you have created the animation use the share button
get a link
copy the link
Now the magic happens
3. In your Google Site, click on the header / select image
Navigate to your albums and select your Animation (gif) as your site header
And there you have it, an animated Google Site Header