This is the code I have used o mine and it works fine. Just paste it at the top of your style sheet. You can see the result here:
http://www.lancemoreland.com body table td a img { display:none; }
.thumbnails table img { display:none; }
table img { display:block; }
.display,.display img { display:block; }
.thumbnails table img, .display,p.small { display: inline; }
body div.galleryheader table td a img { display:inline; }