Do you mean the signature at the bottom ? If so check out The Signature Shop under the forum called TCS Support/Announcements - theres lots of hints and tips there - also if you are a member of a web-site that hosts photos - you can design and import your own. Also we have some great signature designers - again in The Signature Shop you can post a request for someone to make you one, you just post links to your photos.
You can insert it through the User CP (link is at the top of the page) and then click on Edit Signature, you have to be a member for 24 hours, so it may not show right away.
If you have an image (it must be less than 150px in height and no wider than 640px and under 30kb in file size) you can upload it to photobucket, or other hosting site and insert it into the 'edit signature' box.
If you do not have an image you can use, you can request one in the requests thread in the signature forum