The image IS inline. (I guess what you mean is the choice you get when clicking on the image in the Pages document; you can choose between - in Danish - "integreret" og "flydende", guess that equals "inline" and "floating" ...)
I have tried importing styles from the document mentioned, I have tried the two methods mentioned in that document, I have tried creating a new document from a template and pasting my document in.
No matter what I do, "ePub Checker" gives me the above mentioned error message.
But maybe "ePub Checker" checks something else than the end product in "iBooks" ...
Cause actually the file HAS been made available (free) in iBooks store a month ago with no problems.
But then I decided to sell it for money in the Danish eBooks-stores, and it has got to "pass" the 'exam' made by this "ePub Checker".