Rui CarmoEtherPEG is a free program for Mac OS X that shows you all the JPEGs (and GIFs) going by on your network:
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.
It is amazingly effective as a demonstration of the utter lack of Wi-Fi security - people tend to understand the impact of sniffing a lot better when they can see it in an easy to understand form.