The section named "Signatures stored in files" in that last link explains how to use Thunderbird itself to compose a signature. Use the Insert|Image menu item to insert an image. The image properties allow you to make that image into a clickable link. A more geeky way to do it is to insert a link then add the picture as the link text. But that requires you to hand-edit html code which I recognize doesn't appeal to you. But I'd keep it simple and just add a link using Insert|Link. The best signatures are mainly text with a minimum of graphic content. They have best chance of surviving and delivering content despite anti spam measures. And do think about what your signature will look like to users who view their email in plain text only.