click on User CP, scroll down to where it says edit signature, click on that, then put whatever text/images you want in the box. You'll get an error message if you put too many photos, or too much text ...
Crikey - OP and I must be twins or something... Just thinking exactly that and found that the below looks really messy - how do you make those lovely banner thingeys with all the little pictures side by side all nice and neat......?