A Scanner Darkly - VB.NET image

Looking for a posterize algorithm for jpeg images. Ideally, I am trying to make some images into a comic book style like "A scanner darkly" Here is an example of what I am going for: [login to view URL] Here is some code that has a posterize function that isn't quite working for me. [login to view URL] I am wanting to have the code that does this rather than doing it with photoshop or some other tool. I can provide a VB.NET interface with a loop ready to go through each pixel but it is the rest of the algorithm I am in need of. I want the source code, more than the application. I would like to incorporate it into something I am already building.