www.photobucket.com
www.imageshack.us
Go to one of those sites. Photobucket requires that you register. Imageshack doesn't.
Upon uploading to imageshack, you'll see the direct link to the image somewhere on the page, usually at the bottom. Put that inside
tags and it should appear in your sig.
Photobucket is easier since it gives you the code required for Forums. It appears as
. Simply copy and paste it.