I've heard of pictures getting returned because there was no name written on the back - that was for pictures of dependants. If it's just one applicant, I don't think they would make an issue of something like this. They are not automated machines, they are people. They do not necessarily reject something just because it's a little different in the instructions.
That said, maybe for some reason they need a photo with an empty back, so ... *shrugs* If the application hasn't been sent yet, I would redo the photos. If for no other reason than to save yourself the constant doubt and concern that your application will be sent back. If it's already on its way, sit tight and cross your fingers
Usually, if the photos aren't ok, the application is returned before the sponsorship is granted.