When reading the information in ePub documents from Apple http://support.apple.com/kb/HT4168 this is passage to be noted
Note that some content is not supported by the ePub file format standard and will be removed from your document when exporting to ePub:
- Master/Background objects.
- Headers and footers.
- Floating graphics.
- Footnotes (converted to endnotes).
- Some image effects such as shadows.
- Images beyond the 11MB of un-encoded image data allocation per chapter.