@oristarA,
It has gone now; I only noticed it because I have just started using a Firefox add-on called RequestPolicy, which (among other things) shows when a web page contains images hosted on a different site, and blocks them unless I decide otherwise. Maybe Able2know put it there. Or someone else. Web bugs are used to track people. When your browser loads the image, your IP address is visible to the site hosting the invisible image. Remember it was a 1 x 1 pixel transparent GIF file, so invisible ordinarily.