Recently, I had a client that ran into some problems with the cover art for their project. It seems iTunes has some rather stringent (in my opinion) requirements for the cover art. Part of me understands that there needs to be some requirements or checks in place and I agree with Apple that people should not be 'misleading' in the representation of their music. But, I felt this particular case may have been a bit rediculious. For those of you that want to know, here are the specs that I've found.
These things are required and will prevent you from releasing your music on iTunes: