I'm amazed at the variety of photo editors available for the iPhone and iPad. Many are "me too" apps that don't offer many unique features, but Repix (free) offers a fresh idea. It can help you create some very artistic photos with only a little effort, even if you aren't the most creative photographer on the planet.
Repix contains a collection of brushes that you use to paint over your image. Each brush applies a unique effect. For example, there are brushes to "cartoonize" your image, or make it look like charcoal media. There are also brushes for edge enhancement, super-saturated colors, and posterize brushes. Others are erase tools that can clean up unwanted changes (there's an undo command, too).
Repix has access to your camera, so you can take an image from within the app. Edit tools include crop, saturation, color balance, temperature and vignetting.
The app works quickly, and saves at full resolution of 4,096 x 4,096 for the iPhone 5 and iPad 3+. Help is included, so I doubt anyone will get lost using the app.
Of course, Repix's developers would like you to add more brushes, and a Master's Collection is available via in-app purchase for US$4.99. I think that trip to the wallet can be avoided, but if you really get into using the app you might want more options. You can also mix and match brush purchases, so you could just get a set of Light Brushes (stars, streaks, etc.), for only $1.99. Other combinations are offered.