Problem with ebooks with pictures

I bought a book for my son with loads of pictures (a book by Terry Denton and Andy Griffith). The book is automatically synchronized with my own ereader (kobo clara) and works fine.

For my sons ereader i download the book, the file is automatically made into a epub by digital editions and I place it on his ereader manually with digital editions.

On his ereader (also a kobo, a kobo nia) the book does not show any text, pages are cut off and some pages are even empty. The same happens when I borough books from the library. I place them on the ereader with digitial editions and the books not readable. This happens on both the ereaders.

As the books downloaded through the kobo shop work fine, it should work.

Is anybody familiar with this problem and how can I solve this?