Well, that did not work. And finally I think I get it.
I've had that happen a couple of times when I've just not taken a picture of a plant or lost my photo, and had the same issue. I have resorted to just providing the address of the photo in the database in the comments section.
In order to add one of those database photos to your listing you would have to download the photo to your computer and then add it to the listing. Each photo's details should indicate what kind of licensing the contributor assigned to their photos. Would that matter if you are using it within this site? Dunno.