Picsart, a not unpopular image editing app, is available for Windows Phone for some time already. Besides basic features like resizing and cropping images you even get Photoshop features like the clone tool. You can also draw inside pictures, use clipart or masks, add text or borders. In terms of features it is probably one of the most advanced photo editing software on Windows Phone, but it is also a fast growing social network.
The app has a nice look and works very fast. You can find it here for free.