You can insert an animated GIF image as a background image or banner image.
This involves using the animation feature in Google Photos. Instructions in Google Photos:
Make animations & collages
An animation is a group of photos put together to create a photo of a moving subject. A collage is a group of photos combined into one photo.
On your computer, go to photos.google.com.
Sign in to your Google Account.
In the left menu, click Utilities.
Under "Create new", click Animation or Collage.
Choose the photos you want to include.
At the top, click Create.
You can also create animations and collages by multi-selecting photos from your photo grid.
To make a slower animation you can use an editor like LunaPic. You can upload your fast GIF, change the speed, save the image and upload it to Google Photos, get its URL and insert it into your page. (Don't forget to move it into a shared album). LunaPic can also be used for Video to Gif converting.
See this example
Also see the image page .