An additional question:
I have a PRC generated by Mobi Reader 6.x beta (from aPDF). The prc does actually contain the cover art as the first image resource. So I patched 201, 202 and 203 in to the EXTH. Set its data to NULL, and expected the gif to show up as cover art in Mobireader. Well. That is not the case. I wonder if MobiReader expects the thmb to be a certain size max and I can not test on a Cybook :-(. Is there anything else that might need to get set ?