I saw this could be done using Linux. It requires Mozilla, a desktop like Gnome or KDE (not sure which one in particular), you had to have a particular printer driver, and an image tool set to convert the printer output to an image.
Its been a while since I last looked this up and I'm not sure how accurate that is. But basically you have PHP issue a shell command which fires up Mozilla from the desktop. In the command, you point Mozilla to the website and have it print the page to disk. Then you use another program to convert the printer output (I'm thinking its in a flavor of postscript) to an image.
Google search and you'll find more info...