The memorial was meant to celebrate Caylee's life. Casey was her mother. That can't be changed. I think it's important that people see that there was love (or at least what appeared to be love) shared between Caylee and Casey. Had there been huge photos of Casey all over the place I would feel differently, but one small photo in which Caylee was obviously the focus of the photo is not inappropriate, IMO.
I believe that photo was on the table right beside the huge heart floral arrangement with Caylee's picture in the middle that was shown over&over so it wasn't just a small photo that nobody would notice.