With version 4 I got Multiformat book not supported on the Cybook. But version 4 worked on my T5. And on my T5 my generated file with the 600x800 image works.
A bit hard to debug when the error only is on the Cybook.
I attach the file that crashes my Cybook but works on my T5. It would be nice to know it the file works or not on other readers (you cannot upload .mobi files...)