I had a steam sig once, but then it stopped working. At first I thought it was just my own but seems it happened to everyone. Site shut down, or got canceled.
So I got a new one from a different site. If you still want a new one you can make one using any of the following sites. I'm currently using the first one. The second one is more customizable but you need Microsoft Silverlight (using the non-Silverlight option is confusing as shit). Nothing special about the third one but it does its job.
http://steamcard.com/
http://steamsig.net/
http://www.steamsigs.com/