Thank you for commenting, Dave, even though that's not what happened. You're the only one who's at least tried to help me figure this out.
No, it was the first time I'd added Justin's photo to his profile. It never worked from the beginning. I would like to try deleting the 3 attempts and start over, but am not even allowed to do that since the site says no images were ever uploaded!
I have done what you said for images of my grandmother and her sisters, and will probably do that a lot more (add other profiles to the existing photo.) But Justin was only the 2nd nephew/niece whose photo I was adding, and the other 7 uploaded just fine.
It makes no sense, but must be a technical glitch that only a WikiTree I.T. guy can help with. However... I can find nobody who fits the job! Any ideas there?