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You can use any image, just drag it to the window, if its too big it'll ask you which parts you want to crop out.

Here's mine, I got it from a music video:

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i have no idea where ichat stores your icon. but you can just click on the icon box on your buddy list and paste any image in, and resize it accordingly:)
 
I've had trouble making this work; for some screen names, iChat insists on using whatever icon it first used. iChat for me has been, on the whole, unreliable.

I get a BA in a week! Yay!
 
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