Firstly, you have to upload your picture to a server--you could use something like
Image Shack, or
Tiny Pic--but
make sure that your picture is below, or at, 600x120 pixels (600=horizontal, 120=vertical).
Next, go to your
Forum Profile, and look at the signature field. From there, copy and paste the URL. For practical purposes, I will use an image of my own. Then, put image tags (shown below in the code box) around the image link, like so:
[img]http://img73.imageshack.us/img73/9839/bannersizecopcopy9zq.jpg[/img]
And that's it. Your image will show up beneath all of your new posts (it doesn't update your posts made
before you change your signature).
EDIT: For reference: Make sure your signature is under 200 characters, since it doesn't allow more than that. Also, if you want to make it linked, use the URL tags, like so:
[url=YOURURLHERE][img]http://img73.imageshack.us/img73/9839/bannersizecopcopy9zq.jpg[/img][/url]
And there you go.
Evil has a new name. Demo out now!