Do you mean the image signature that's hanging about under your messages/posts?
If that is so, you can do this step:
- Find first a suitable image/ the one you liek for your siggy
- Get the url (you can do so by right-clicking it and see image properties. Check the one with the URL : http:// ....
- Afterwards, post the code under your signature box (profile page)
- Then put this code on the first and last portion of your code
----> It should look like this:

- Save your changes
and there you go! ^_^