EtherPEG is a free program for the Macintosh that shows you all the JPEGs (and GIFs) going by on your network.

EtherPEG was written at MacHack 2000 by Sam Bushell, Peter Bierman and Stuart Cheshire.

EtherPEG works by capturing unencrypted TCP packets off your local network, collecting packets into groups based on TCP connection (determined from source IP address, destination IP address, source TCP port and destination TCP port), reassembling those packets into order based on TCP sequence number, and then scanning the resulting data for byte sequences that suggest the presence of JPEG or GIF data.