I grabbed the Coding Horrors bitmap from the free chapter of McConnell's Code Complete 2 at the asp.netPRO website.
I think this picture strikes me as the funniest in all computer books - it's so dead-on. When I run into ugly code, the image instantly pops into my head. The only way it could be funnier is if the hair was animated like an old Scooby-doo cartoon.
My second favorite is the "recursive head-to-keyboard" animated image I've used in blogging before.
There are quite a few in Jim McCarhty's book that I like, but the images are more than a simple little bitmap so I didn't include them here.
Does anyone have other images that hit home?