iBooks: my ePub doesn't show PNG images, a question mark is in its place instead

I'm making by myself an ePub book. In this book I put some screenshot as a PNG image inside an XHTML page. So in the content.opf I referenced the XHTML page...

Until few days ago it worked: the XHTML with PNG image was showed.

Now I see only a blue question mark at the place of the PNG image preview. But if I double tap onto the question mark, the preview correctly shows the PNG image.

This happens in IBooks and Stanza, but not in GoodReader and others eBook readers...

An tips?

(sorry for cross post)