You have to edit your signature for this. It's found in the User CP. Then as Xanthe said you use the image tags to put in your signature. If you click on the image icon (yellow one next to the hyperlink) you can just put in an image link, but doing it manually, it's just image tags.
All images work the same in this case, whether they be gif, png etc.
Your code would be like this.
[IMG]http://dl.dropbox.com/u/35499431/gifs/tumblr_leq423YnIE1qfdropo1_500.gif[/IMG]
Which would give you:
You must be registered for see images
Just replace the image within the img tags with your image.
