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I'd go with paint if you're any good with it
I was probably going to use colored pencils myself. Keep it simple, make the pixel count high and then shrink the image and it should work fine. At this point, I'm not sure I'm going to have the time though.
Tangent & Funky Name Brother; Founder of the Gutter Brethren; Frequent Victim of GirlCo [flickr-photo:id=3958341944,size=s]