Driftnet is a program which listens to network traffic and picks out images from TCP streams it observes.
The program promiscuously sniffs and decodes any JPEG downloaded by anyone on your LAN, displaying it in an attractive, ever changing mosaic of fluffy kittens, oversized navigation buttons, and blurred images of Big Brother Elizabeth fiddling. It's UNIX only. Your sysadmin is undoubtedly running it already. So stop that. Now.