I usually make mine by experimenting with Photoshop. if you have PS, check out www.good-tutorials.com. The one i'm using currently was very easy to make. I screenshotted the Yahoo homepage, and just got rid of the Yahoo text, then downloaded the yahoo font and typed my username
Upload the image with http://imageshack.us/ then goto your "user cp" tab then edit profile, then where it says signiture, copy and paste the text from "Hotlinks for forums 1"(from imageshack) into it.