You would need to edit the file and change the overlay to some other overlay image
$mthumb = "<img src=\"{$url_path}/images/
overlay-roundp.gif\" height=\"105\" width=\"81\" border=\"0\" alt=\"\" />";
else
$mthumb = "<img src=\"{$url_path}/images/
overlay-round.gif\" height=\"81\" width=\"105\" border=\"0\" alt=\"\" />";