You may have also noticed that if you try to upload one, you are more than likely going to get an error page:
If you scroll all the way down to the Administration Forum (which I never even knew existed until a few minutes ago), you will see that gnar started a thread on this problem on December 22nd, 2010. Exactly 8 months later, we still have no solution, even though there are 7 pages of complaints in gnars thread.
We want our dimensions and we want them now! 80x80 is acceptable, but 100x100 would be better. Even 120x120, if the programmers are man enough.