I got a text message with a picture from someone I know. I then received a reply to her from someone else. I keep getting that same reply over and over sent to my phone at all hours of the day and night. How can I stop this from happening?

If it's an iMessage, when iOS 7 is released on the 18th you'll be able to block the sender.

I see. Until you can find a permanent solution it might be worth a try to temporarily block her when this happens, then remove the block to see if the message stops sending. Not sure what else to suggest.

Apple needs an "unsubscribe" option from iMessage, so you can cease to take part of the conversation, just like you would with notifications in a forum. This happens all the time with my family, a group text gets sent out, and you're stuck for hours hearing the beeps of messages getting sent back & forth. If not unsubscribed, just silence it so you still get a record of it, but you stop being notified every time a message shows up from that thread for a certain "period", say like 6 hours (in case anyone reuses the thread to send another group message later).

The sender needs to see if it's stuck in the sending mode on her device.