I did these last night. I was inspired by
cignoh's latest avatar attempt and wanted to see if I could achieve the same effect.
The steps I followed (using GIMP) were:
(1) load my picture
(2) create a blank second layer
(3) trace the major lines in my picture (this is a little harder than it looks)
(4) coarsely pixelize (filters -> blur -> pixelize) the original image - nice big blocky pixels
(5) sampled colours from the pixelized image to floodfill the areas in my traced image
Over all, it worked a lot better than my attempt last year at cartoonifying.
However, I still though there should be a more mechanical way (i.e. avoiding having me draw / trace stuff) of doing it. So I played a little more. One thing I did was to (1) create duplicate layers of the image, (2) seriously blur one of the image, (3) do a difference of images, (4) invert the image. This resulted in a nice image as well.
eclectic, avuncular dilettante 