I've never been a gp on upload duty, but I think it's common sense that you aren't the only person trying to upload a file. They probably want to accept as many uploads as possible, so people can get their customs fast. I don't think they have to explain every little detail as to why a yoshi head can't be uploaded.

If it's not that, then they usually tell you why. "File size is too big/small", "Transparency issues"...
Most people don't know how to fix their "solid black/white" issue. Maybe staff should just make a guide relating to the reasons why they don't accept a file.