There are a few spots in OS X applications where you have the ability to choose a photo to use as a representation of you–Address Book and iChat are two that come to mind. In Address Book, click on your entry, click on Edit, then double-click on the picture box next to your name.
This brings up the picture selection window, where you can take a picture with a connected camera, insert an image from a file, or paste an image from the clipboard. While these features may be well known, this one may not be: you can also rotate any image, once it appears in this box.
To do so, just hold down the Option key and then click-and-drag with your mouse or trackpad. As you drag with the Option key held down, you’ll see the image rotate, along with an overlaid clock face of sorts (see picture at left) to help you align the rotated image.
You can’t use your multitouch trackpad’s twist gesture, though–only the Option key with click-and-drag will work. This trick, pointed out by an anonymous Mac OS X Hints contributor, should work in any program that uses this standard picture-choosing interface.