Well, you get a pic hosted, either host it yourself on some webspace like boomspeed, or maybe someone here will help you out. Then, you just put the image URL in your sig, along with the necessary tags to make it visible to posters.
Edit- clarification, www.boompseed.com allows for remote image hosting, as do some ISPs, if they give you free webspace. I'm sure some folks here know more about who does and doesn't allow it than I do, but boomspeed should suffice for a sig image.
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