Ok to start with you need a picture available on the web. If you have digital image on your computer you'd like to use you can sign up on
www.photobucket.com It's free and very easy to use.
Now goto edit your profile and under the signature section
(if you used photbucket highlight the line under your picture that starts with [img] then copy that line of code and paste it in the signature section of your profile)
If it's a picture from somewhere else right click on the picture you'll see an information box pop up.
You'll see a line that says "address" starting with the "www" part highlight the entire line including the ".jpg" ".bmp" ".gif" etc.
Now in the signature section of your profile type in
[img]
then paste the line of code you just copied
example
[img]yourcopiedcodehere
and close it off with
[/img]
example
[img] yourcodecopiedhere [/img]
this also works for posting various pics into the topic.