Sadly not possible with Javascript/css as far as I'm aware. The animated profile pictures still have a .jpg url. Semi-solution I came up with is hiding images when you hover over them, not great but better than hiding them manually. https://greasyfork.org/en/scripts/39058-hide-images-on-hover
Updated the script, changed it so you have to press delete when you hover over an avatar so you can hide multiple avatars at once.
Semi-solution I came up with is hiding images when you hover over them, not great but better than hiding them manually.
https://greasyfork.org/en/scripts/39058-hide-images-on-hover