Go to user cp, editprofile, then in the signature box put the url which you're sig is hosted at with [.img] before it and [./img] after it. For example: [.img]http://members.lycos.co.uk/jaajoe/hpbimg/samuraipizzacatsig.jpg[./img]
Just remove the periods. And if you don't have a host try
boomspeed
Hope i've helped.