There are very few images of severed heads on stock photography sites. However, there are countless smiling pretty people. So I found one that would work and chopped his head off. I particularly like the graphical way I represented the neck-wound. One of my more successful book covers on a micro-budget.
By the way, Chris F. Holm is a fine writer and pretty swell guy all around. You should pick up Dead Harvest or any novel in The Collector series.