it has to be small. Go to google and search for, say, obelisk the tormentor or something like that. Make sure you're on image search. Then, in the upper-right hand corner you should see something like "All sizes-Large-Medium-Small" Click on small and find one that's less than 7 KB. Save it, then go to Profile on Supercheats and Browse then click on it. Then select "Submit" and there you go. It also has to be a certain kind of picture, too. If you go to google it shouldn't be a problem, though.
General, if you're not using it, could I have the army for a few days?