Signatures are images displayed underneath your post, avatars are displayed at the side, above your post count...
This will (I hope) help new members to the site:
Attaching a signature..
step 1) choose a picture thats a suitable size, anything that stretches the page isn't suitable.
step 2) find somewhere to upload it to if you have your own webspace, upload it there, alternatively use http://www.photobucket.com or http://imgur.com, there are millions more sites like this.
step 3) once uploaded, copy the URL of the image.
step 4) if you want it as your signature go to profile>signature, post the URL inbetween these tags
e.g [img]the%20URL%20of%20your%20desired%20pic%20here[/img]
Attaching an avatar..
follow steps 1-3 then instead of copying the URL into the signature box, copy it into the link to off site avatar box.
If you have any other problems, please contact your nearest MODERATOR who will be more than delighted to help.