The Messages app got a complete overhaul in iOS 7 and the changes weren't limited to just visual ones. One of the best new features of the Messages app is the ability to view all images you've received and sent in a thread in one easy to access place. Here's how:
That's it. You can then tap into and view any image that you've sent or received from that person, or group of people.
As a side note, this view images screen only loads anything that's in the current loaded conversation. If you have lots of messages between you and that person, you can view even more images in that thread by performing these steps inside that message thread:
Repeat the above process until you're either at the beginning of the conversation or until you feel the images you're trying to find would be contained in the loaded conversation. Then return to the view all images screen by tapping into any image and hitting the menu button again.