Yeah, I was gonna say in the other thread "You should get a nice drawing pad".
I used
http://www.inkscape.org/
to trace the outlines. You should get that, it's open source/freeware. It's to adobe illustrator what gimp is to photoshop.
Mainly use the pen tool to trace it and add fills or strokes. The pen is easy to use once you get it down.
Gimp has a pen tool also, but it's raster (pixels)
It's cool 'cause once you trace it you can easily change the stroke colors/ widths and fill colors/gradients. It's vector so there are no pixels (until you save as png) and you can scale it to any size at any time and not change resolution. (I made that other pretty large, exported a png, opened in gimp to save as jpg, then choose resolution options and shrink as needed).
Inkscape is good for lines, solid colors, really clean illustration.
Gimp is good for photos, sprays, more loose airbrushy stuff.
Together they are a great package that let you do about anything.
(there are web sites too that let you upload the svg from Inkscape and add it to a t-shirt/poster to buy/sell.)