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After you have reached that point, then you can use the "URL" tag to add the link to your signature. I tried to figure out a way that you could link it to an image file on the forum, but it doesn't look like that is possible. The only way to link it to an image would be via an actual HTML or PHP page hosted on your own site.
If you would like to see how that works, just let me know and I will add a 3rd video showing you how to do that.