https://matrix.gvid.tv/settings
There you should see an option: "Prevent animated thumbnails"
Personally I don't mind them. To me it's a good idea to keep the default on because I think it does a good job to communicate to first time visitors that this site isn't exactly like every other reddit clone really at first glance and also feeds them that first micro-dopamine hit.
But in any case it can always be turned off.