Adding an animated green screen background is simple, just go to the "green screen" set up in your event on the SNAPPIC online portal and instead of adding an image, you can now add a video which loops in the background.
You can upload multiple videos for your guests to choose from.
The video must be
Less than 10 MB,
720p = 1280 x 720 (or the same aspect ratio as the template you have uploaded)
A couple seconds long
It must also be a loopable video, ideally but not required.
Videezy is a good resource for free stock video footage.
Alternatively, you can also create videos with images to create some custom animations. Adapter is a great tool and it is free!