It should take 100x100. Are you doing it through the My Account link here on the GC2 forums?
There seems to be two ways of doing it.
1) On the home page at the top there are two clickable things....'My Posts' and 'My Account'. If I click on 'My Account' and then 'update images' it reads that only 50x50 are allowed and thats all it will let me put in.
2) If I click 'My Posts' and then 'My Account' then 'Images' it reads that 100x100 are allowed and I can put that in.
On option 1 the image of 50x50 says its an 'Icon' which shows up when you post. Option 2 says its an avatar which also shows up when you post.
Whats the difference? Am I missing something?
One more thing. What does the image option for 'Desktop Screen Shot' do?