-- Update --

I found the site where I first saw this.  Turns out he wrote a tutorial.
You can check it out at: http://www.themanwhofellasleep.com/invisibilia_tutorial.html
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I love my new "Coloring" book pictures, and though I'd share the skillz.
Open your fave photo editor, I use Photoshop, though if you need a free programThe GIMP works and I think Paint.NET works too.
The important thing is that the editor can have semi-opaque layers.

So, now that it's open, make a new layer and fill it in white. Drop the opacity on that layer to around 35-50Ã Grab the paintbrush at around 1-3px and trace.
Then select the remaining outer portion with the magic wand and delete.

Viola! I'm sure this would be easier to understand with pictures, if someone really want to know how I'll take them.