Adobe PS Touch is a much more advanced Photoshop like editor for your images. You can combine images, use layers, selections, adjustments and apply professional effects to your images While there are not masks in the app yet, you can use the Scribble Selection tool to select parts of an image to extract them. You can even use Refine Edge, to refine the edges of that selection. At this point you can work on high quality image up to 12 megapixels. I am sure they will add updates as needed to keep up with phone camera output. There are built in step-by-step tutorials to learn the tools and techniques on their use. These stay with the app so you can always go back for a refresher when ever you need to. You can open your layered PS Touch files in Photoshop CS6 & CS5.
You also get a free membership to Adobe’s Creative Cloud. You’ll gives you 2GB of cloud storage plus the ability to sync between tablet and desktop, access files from the web, and easily share with others it you want it to use it.
Note: This app requires an iPad 2 or later and iOS 5, it is not available for 1st generation iPad.
This is not a free app and is on the higher side as far as apps go, but it is a full featured editor.
Check it out at the App store: https://itunes.apple.com/us/app/adobe-photoshop-touch/id495716481?mt=8
Check out the Adobe website to find out more: http://www.adobe.com/products/photoshop-touch.html