I'm not sure about Signature Images, the other Board I frequent also requires Signature Images to be hosted elsewhere and included using the Insert Image URL button from the row above the text box...
What's weird is that I've just copied the line from my Signature in my Control Panel and it's appeared here as a different size to the Signature version....
But what we don't have here, or in the new version at present, is the ability to include Avatars.
I shall drop an email to Simon about them.