Try re-uploading it after three or four hours, so that it's past the reviewing admin's bedtime. Some one else might allow it. Alternatively, search for related threads in this subforum, post the image you're trying to upload in this thread, and be frustrated for days trying to make it work.

Apparently Jarace (admin) deals with this in a human manner (without spam responses) if you catch him in person, but I rarely see him in game.