I have read the posts and I still don't really get it. First cover was rejected because:
The cover art file that you upload as part of the book asset in the Assets tab in iTunes Producer must be at least 1400 pixels along the shorter axis, must be in PNG or JPEG format, and use RGB color mode. For best results, we highly recommend 2400 pixels along the shorter axis. Please do not increase the size of a smaller image to meet the minimum size requirement as excessively blurry or pixelated images will be rejected.
So I read that to say the horizontal axis of the book cover is > 1400 pixels - which nmeans the vertical axis will almost always be more. OK, so I modified the files, renamed them, deleted the old cover and tried to select the new version and the file was grayed out. Why would a JPEG (and I tried PNG) be gray - not allowing their selection?