You need to put your jpeg together first. If you need something to build on, check here: http://techist.com/showthread.php?threadid=68745
JuiceDaddy put together a pretty good compilation of saig basics... blanks, if you will.
Anyway, once you have made your sig, host it online somewhere. I like using ImageShack (http://imageshack.us/).
Once you have it hosted, just copy and paste the URL in the appropriate spot in your User Control Panel.
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It seems JuiceDaddy also has instructions posted in the OP of the thread... Just ignore mine if they conflict with his.