There are so many photo-editing apps in the App Store it's hard to keep up, but I want to call PhotoPath to your attention. It's a free, universal app that can be, for many photographers, a one-stop solution for creating multi-photo presentations.
The app starts with a series of frame layouts. Pick the one you like, then tap on each frame to either add a photo from your camera roll or shoot one with the camera. You can scale the photo and move it around until you are happy with the composition. Then add some filtering like black and white, sepia or intensify the colors.
PhotoPath allows you to change the type of frame and borders, as well as adjust the size, color and border design. You can add a caption, which many photo apps don't include. It's a really nice app to have when you are sending images back from a vacation or party, because you get something "finished" and not just a series of photos. You can save the finished project to your camera roll, or email it. You can also print from your iPhone or iPad if you have a compatible printer.