Yesterday, I wrote about Luminance, a streamlined-yet-versatile photo editor that I think iPad users in particular will like. Paint FX ($1.99) is a bit more complicated, but for the slightly steeper learning curve, you gain the ability to edit only select areas of a photo.
To get started, tap the load/share button in the upper-left corner to load a photo from your Camera Roll, Photo Stream, or iPhoto (if you have your iDevice set up to sync some portion of your iPhoto library). You can also grab a photo from Facebook. This button also lets you save photos to your Camera Roll or share them via Facebook, e-mail, or the Paint FX community page.
When you have loaded a photo in Paint FX, tap the FX button to select an effect. By my count, there are 69 filters or effects. By tapping the small Paint FX or Fill FX buttons below the belt of effects, you can choose to apply an effect to a portion of the photo or the entire photo. If you choose Paint FX, you will then paint with your fingertip to apply the effect. You can undo and redo changes using the back or forward arrow buttons in the upper-left corner. You can also tap the Erase button among the row of buttons in the upper-right to erase an errant edit.
The middle button in the row of buttons in the upper-right corner lets you edit your brush; you can adjust the sharpness of your edge, the opacity, size, and other attributes of your chosen effect. Each effect has its own set of attributes. To make fine edits, you can pinch to zoom and then reposition the photo by dragging with two fingers to zero in on an area of a photo.
You can add multiple effects to a photo by tapping the Layer button in the upper-right corner. This keeps you current effect and lets you layer another on top. If you don't select a new layer, then you'll just replace your current edit with a new one.
The Mask button is also very useful. It lets you try out different effects to find the one you like best. Instead of painting an effect on your photo, you can tap the Mask button, which lets you use your fingertip to highlight an area of the photo. After you have highlighted an area, you can then tap the FX button and select one effect after, saving you from having to redraw the area you'd like to edit each time.
If this sounds a bit confusing, there are two helpful videos located in the Learn More section of the Paint FX app. Tap the question-mark icon in the upper-right corner and select Videos.