Open iPhoto and choose a photo that you'd like to tweak. Tap Edit, and then tap Brushes. Choose a brush—for this example, I chose saturate—and then tap the Edge Detection icon in the bottom right. Now tap the gear icon next to it to call up the brush controls. The second option will be Show Strokes; turn it on and then tap anywhere in your image to make the brush controls pop-over disappear.
Place your finger in the center of an area with a well-defined edge—enlarge the photo if necessary--and then slide your finger across it. As you do, you'll see that you're highlighting it in red. Continue to slide your finger around, highlighting the entire area you want to edit. Thanks to iPhoto's Edge Detection, you'll notice that the red highlight does not follow you if you drag your finger across the object's edge. (And if you happen to get a few errant brush strokes, remember that you can get rid of them using Erase.)
Once you've highlighted everything you want to edit, tap the gear icon to return to the brush controls and turn off Show Strokes. You'll immediately see the results of your efforts; if the edit is too strong, simply use the slider at the top of that window to reduce the intensity.
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