sorry...i though that i posted, but now realised that it no there..
here is a way, give it a try!
Scroll down to "Signature" area and type:
To include an image into your signature, go to "profile" in the top blue bar
Scroll down to "Personal Signature" area and type:
< IMG SRC="
http://yourwebsiteaddress.com/image.jpg" WIDTH="***" HEIGHT="***" >
Remove the space after and before the brackets.
*** can be replaced by the image resolution.
I tried, it worked for me!