Pixelmator has been a long-time favorite of Mac users wanting a less expensive but incredibly powerful alternative to Adobe Photoshop. It's beautifully designed, well supported, and powerful enough to handle even complex image editing needs.
Now Pixelmator has come to the iPad, and it's brilliant. Designed for iOS 8, it incorporates a number of exclusive features like Extensions, iCloud Drive, and even Handoff, which allows you to start working on your iPad and continue working on your Mac--and vice versa. It's a 64-bit app, designed to take advantage of the tremendous processing and graphics horsepower in Apple's latest tablets--just like the Mac version.
Lastly, Pixelmatr is compatible with Photoshop, which means you can save Pixelmator images as Photoshop files, and even open Photoshop files in Pixelmator. Most of the popular Photoshop features are supported, so collaborating with Photoshop users is no longer a concern.
If you have any interest in doing serious image editing on an iPad, this is undoubtedly the best way to go about it. It's available now at the stunningly low price of $4.99, which makes it a no brainer for just about anyone.