Go to the 'settings' option by your profile, and in the signature box put [img src="URL to image here"] but replace [] with <>. Use the preview button to see if everything works properly beforehand. From the sig/av thread, they recommend a size around 400x150.

Here's the HTML FAQ if you need any more help.

Go to "General Discussion" on the website. That's where you'll find your answer for how to do that.