You Doodle has been downloaded over a million times and is the only iOS image editor you will ever need. The ever growing list of features, seems to grow. Being able to share a photo or drawing to Dropbox, Facebook or Instagram has a convince with mentioning.
When you are able to have a small sketchbook saved within an app and also use it as a photo editor, that is something to look at! And since there have been more features added, go through the list to see what would fit you best.