Rob, your first impulse was correct. Commenting on the photo is the best way of handling this. It starts a thread in the Plant Photos Forum, which is watched by all of the database admins and by many plant-savvy members, and it automatically sends a message to the owner of the photo. If you were to simply propose that the image be moved, the photo owner would not be notified, and it's likely that your proposal would be declined and you'd be asked to post a comment on the photo instead.
In this case, however, the photo owner is growitall, who is no longer active on the site, so her imput isn't necessary for the move.