The autoresizing system should work for any photo posted as well as avatars. The max size an avatar can be is 64x64 (which is the standard avatar size). This makes sure all the forums look uniform.
I have changed the max avatar size from 20k to 30k (your image was 23k or so) so that may have been the problem.
The only other problem is that the size would make the pic look wierd - as it is very long.
**UPDATE** - I just tested with your animated pic with the new settings and it accepted it fine (and looked quite good actually) so yeah let me know if you have any problems but it looks like it should now work fine Jayse.