I open the content of the ePub file, and I think that the problem is the accent in the name of the image. Maybe can be this?
Edit: That's the problem, the accent. I remove the accent and now the image appear again. Not as cover but I can see the image again
The OpenOffice 3.2.1 in spanish put that name by default.The original name of the image has no accent. This is why I don't noticed before.