first you have to host the image somewhere so you dont steal the bandwidth of other sites...then you have to link the image with image tags. for example the coding of my signature is:
[ CENTER][ url=www.med.monash.edu][ IMG]http://users.tpg.com.au/adsl353v/logo.gif[ /IMG][ /url]
[ url=http://johnhoward.blogspot.com/][ IMG]http://www.geocities.com/inasero/pmposse.JPG[ /IMG][ /url][ /CENTER]
center means i want to center the picture...then the rest is pretty intuitive