What exactly does it say when you try to upload? I know the most common problem in the past has been people changing bmp to jpg. This does not count as saving as png.
I use png(s). It doesn't say anything, it just doesn't upload. However one time it told me I already had too many screenies and that I needed to log out and log back in.
I know sometimes I choose the file, type the message, and click "Post!"... without clicking the "Upload Screenshot" button first. Though, I doubt that's your problem.
