Alright, I'm back with an answer. Converting it to a JPEG worked. I'm suspecting that the problem was that I had shadow/transparency in the banner and the site couldn't quite accept the formatting, but if you are having this problem, convert to a JPEG.
It's exactly what the size is listed as, 468x60 within the size requirements and an accepted format. I don't really think I want to bother the mods, who are probably busy enough as is, but if it's the only option I have left...
I'm going to try converting it to a JPEG, then I'll check.