Sorry, I misunderstood you.
You would have to resize them appropriately. As they would become stretched in the process, they won't look good after, though. Much easier to resize a large picture into an avatar-size, while still retaining good level of detail...
These look like good signatures, though:
http://members.aol.com/zoetko2/newdk.gif
http://www.mm2x.com/upload/files/balrog_sig3.jpg
http://zbom2.free.fr/tcheval/signature/balrog.jpg