I get this message when trying to select my cover image from "MyFiles." I'm not sure what the problem is? It's a pdf and it's definitely the correct size.
My cover pictures are in the JPEG format not the PDF one, and I have never had any problem with them so far. Have you tried uploading a JPEG?
I think you can only upload a PDF for your cover if you are using the one-piece cover designer. Otherwise use image formats.
I'm using the one page cover designer. I've formatted a pdf to the size specificed, and when I try to select it at the one page cover step (it's already uploaded to "My Files"), I keep getting the "oops!" message.
Aha! I just reformatted it as a jpg and it accepted it. Apparently the one-page cover option actually does not work with pdfs... thanks Jean Paul!