First you'll need to upload your picture somewhere on the internet. For instance: tinypic.com or imageshack.us.
Then in your signature you write [img]http://website/urltoimagehere.jpg[/img] and that should work if done properly. Please note that we do have a maximum height for images, and although the rules don't mention a maximum width, if the image is too wide and thus screws with the layout of the forum, I'll be asking to remove/change it too.