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Photoshop is the embodiment of pro image editing, but it's a memory and processor pig. If you'd like a very simple graphics program—one that works completely in the background—Mac OS X has your ticket. It's called Image Events, and AppleScript commands it with ease.
Open the Image Events dictionary (File → Open Dictionary), and browse through the commands in the Image Suite. These commands cover all the manipulations you can make to your images: cropping, scaling, saving in various formats, and so on. The following pages provide just a few examples of what you can do with these commands.