I think the possibility to add images as annotations to PDF files would be useful. Considering the limited space usually available in documents, I think the best implementation would be to add an image as a little note that opens as the full image when clicked and/or hovered, but the option to have the image as is (making it resizable), would be useful too.
This feature is already implemented, though very hidden. *** This bug has been marked as a duplicate of bug 383652 ***