Quick and dirty: Pixelmator is the best image editing software for iOS, period. And that’s a real fact, not some alternative one. Right now it’s on sale on the App Store for a measly 99 cents but that price won’t hang around so you need to get it right now.
Doesn’t matter if you are an average dabbler or an actual design pro, you want Pixelmator. Does everything … image editing, touch-ups, sketching, compositing, talks to your Mac, talks to Photoshop, just quit reading this and go buy it, okay?