The "Paper Camera" style of image, a kinda faux-sketch view of the world, is actually a very simple set of image processing steps:
1. Take an image (this is Franklin bridge in Philadelphia):
2. Find the edges in the image and make them into a dark outline:
3. "Posterize" the brightness to make it look cel-shaded:
You can download this filtering effect as a plugin for ImageJ from my website: here.
ImageJ - Scripting and image processing