First I get the image url, via right-clicking the image and selecting the 'copy image location' or 'copy image URL' option, depending on the browser, then I go to the Signature editor, click the 'Insert/edit Image' Button, paste the url into the box that appears, and hit insert. This should fix your problem, unless you are already doing this, in which I case I have no idea how to fix it. Anyways, hope this helps.
Gamigon
BMPixy