HTML image elements mysteriously disappearing?

Please see here, scroll down to Product Gallery and there should be several large dress images but they are not displaying at all.

When I check the HTML code and they are all there and the images are all correctly showing up if I access them directly in the browser. However, when I use Firebug to examine the elements they seem to be faded out or hidden or something.

I tried to add these CSS styles via Firebug to the elements on .product_images_exhibit img:

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