Convert Image Into a Tile Mosaic

[J_S_R]

I would like to take an RGB image and convert it to a mosaic pattern on a square grid, with each grid a monochrome color. I've tried a couple of different approaches but I'l lost. Any help would be appreciated.

Ideally what I'd like to do is Slice the image into grids. Average Blur each individual grid. Posturize the entire image to a set number of colors (3). Combine all of the slices back into one image. Then make each of the three colors a spot color so that I can replace/adjust as necesarry.

I can slice the image with no problem but I don't know how to Average Blur an individual slice. In the end, once I get the fundamentals, I'm sure it will require some scripting because I may have as many as 400 slices on the image.   

[Chuck Uebele]

Reduce the image to where the pixel count is the number of tiles you want across the image. Then enlarge the image to what you want as a finished product, used nearest neighbor as the algorithm. Then you can reduce the image to 3 colors.