You need to have the picture uploaded on the web somewhere so that it can be linked to first of all. If you're unable to do that then send the image you want to use to Micky at
carrotlots@yahoo.co.uk and he'll be able to send you the link to use.
Once you've got that, then you just go onto the page to edit your signature as normal and then add the link by putting {img}http://....{img} around it, only using square brackets [] rather than round ones.
The image should then display on your sig.