well the issue is that you are trying to post an image in a link which is not something that is normally done. This is a bug actually. I would change the code Lumpy is referencing as well as any other css related to postlink to remove the borders and instead add a text-decoration: underline;
not sure why posting a image as a link would be unusual. it is pretty standard everywhere. It does seem strange that phpbb has this set as a border-bottom instead of the underline bit.