Hi Anne,
I answered your email with the same question, but I'll answer it here, too...in case others have the same question.
Unfortunately, replacing an image isn't an option with our gallery software. There have been a lot of requests for that feature in the xenForo software, but so far, it's not available.
So, the only option is to just delete the old image and upload a new one. Sorry about that. I know it's not as convenient as replacing it.