To send contents of filedocuments (including images) you need to explicitly map the entity filedocument. If you do that, you can expose (or map, if you're importing) a special attribute, "contents", which represents the actual file linked to the filedocument.
Agree with Achiel, though you have still one problem; The image itself has been mapped, but it is not possible to show it as a thumbnail, because this won't be generated automatically. Anyone have a solution for that?