Add ability to add more pictures to better explain book quality

[ItsEcholot]

The ability to add more pictures to the offer could enable the seller to easily provide a better insight about the book quality to the buyer.
This would change the book detail page tough as we would have to display the custom pictures for each offer.

[ItsEcholot]

This would obviously also change the API, although the most important features (image upload) are already implemented.