Sorry for the late reply.
Dunno about a new feature that would prevent animated gifs as avs, will ask Mouse about it.
About the upload file feature, last time I checked an av of -exactly- 64x64 pixels was required for the upload to work. No idea about that 100x100 thing, never tried to upload bigger or smaller avs.
When I want to change my av, I upload it to my webspace and use the corresponding option in the ng.com user cp to download it from there. File uploads don't work well with some browsers, and a number of routers/firewalls also prevent it.