Gary.....Click on the USer CP (control panel) tab at the top of the page, select user profiles, then stick the url tag of the image you want to use in the signature box in 'img' tags.......
(start with square brackets and end with slash square brackets......)
Or did you mean your avatar???
Ekk.....in that case, same procedure but you'll see an avatar selection thingy on the same page...either choose a default one or link to a url for an individual one.....in either of the aove cases you'll have to host the image yourself or find an avatar or piccy that you like and post that url.....
There are many many avatar sites out there....do a search on google and you'll be spoilt for choice.