Yea. I tried to do the same with signatures in hotmail and other ISPs.
But failed.
The best I could do was to insert a picture in my signature here in daniweb. See below....
Though I think it is very much possible to attach cliparts to ur email sign, i dont think u actually can directly upload that from ur pc to ur email accounts signature. What u can try to do is upload that clipart in a free image host like zippyimages.com. Then use a java script that loads that clipart into ur email.

Well i dont know how to code java scripts. Is there any one out there who can help us with this????