Forum is powered by SMF software and it's good to read documents for SMF software for general information. However, theymos customize some forum features and it's no better place to check it than in the forum.
Forum user avatar gives you most up to date information.
Select the avatar you want to upload. It must be no more than 120px wide, 80px tall, and 100 KiB in size. It must be a non-animated PNG, GIF, or JPEG image. It must be safe for work. To remove your avatar, submit this form without choosing any file to upload.
It does not provide all information about accepted images here but about image type, it has enough information.
Animated avatars were disabled since February 2019.
OK, APNGs are now detected. Old ones are grandfathered for now, in part because people have ongoing campaigns, but in the future I might (or might not) go through and remove them.