It has to be a pre-existing costume in your roster, so if it's a new one, then add a new costume (doesn't need pictures) first before you copy pictures over.
I've been trying to figure out why I couldn't update those pictures into the right costume, but that above explains it.
However, I created several costumes in March, three of which were added right away and I could immediately start putting pictures in. The other three I created at the same time are still sitting in the "Waiting for Approval" area. I've gone in several times to update them, thinking that maybe I hadn't saved them correctly or something. Is there a step I'm missing somewhere? Any help would be greatly appreciated!
ETA - I added new ones today. All of those except 2 have been approved. But the ones that I added in March are still sitting there without being approved. I'm just really confused. I'm sorry for the trouble.