I take a photo and open it in Photofiltre 7. That has the option to Posterize, which allows me to create blocks of shapes. I sometimes combine that with creating a pencil image in Picasa 3. I have attached two more examples below, one of which you may recognise! That was taken from a picture of him on the internet.
Hope this helps.
Interesting John. Is Photofiltre 7 totally free?
Yes it is.