What version EPUB did you choose?
version 2.0.1, first page as cover. The file this error occurs is: OEBPS/cover.xhtml
I tried it with a jpg file and a gif, the same error occurs.
Try version 3.
Okay, I’ve tried that. Now i’m getting a warning (not an error):
Message: CSS selector specifies absolute position.
I don’t know what the heck that means...
Did you crack this thing open and do any work on it before trying to validate it?
Nope, it’s exactly the same Indesign file, the only difference I exported it to EPUB 3 in stead of 2. But epub 3 is not always readable on older ereaders, isn’t it?