The recently launched Leonardo is a full-featured image editing app for iOS that offers tools you won't find on too many other comparable apps, especially since PhotoForge was taken off the market when Yahoo purchased it. After a quick run through Leonardo, it seems that this little app may bring some competition for Filterstorm (my personal favorite for mobile editing).
Although there's a slight learning curve to get the hang of the user interface, some of the tools will be familiar to Adobe Photoshop (and Elements) users. Even if you're just starting out, Leonardo offers tutorials and help from within the app. For example, selections can be made using the magic wand or lasso tool. On the iPhone, a magnifier automatically appears when using the lasso tool so you can better see your selection. Layer masking is also available, with options including invert mask and feathering.
Among the editing options you'll also find a trio of gradients (linear, bi-linear and radial), unsharp mask, an adjustable histogram, and a Shadow/Highlights tool that allows you to increase the dynamic range. White balance can be adjusted by temperature and tint sliders and RGB channels can be adjusted with individual sliders. All the basics are there, too, such as cropping, straighten, clones and your EXIF data remains intact (IPTC and Geotag info can be edited). The app can export PSDs at resolutions up to 4096 x 4096 on iPads; iPhone resolution is limited by and equal to the resolution of the built-in camera, although on the iPhone 4, the maximum resolution is 2048 x 2048.
Leonardo requires iOS 5.0 or later and is available now for $1.99. However, the iTunes page suggests this is a "60% off promotion," so if you do a lot of editing on your smart device (or if you want to), now may be the best time to download Leonardo.