Problem with lightbox shading behind photos

Hi,

I’ve noticed a problem with the background shading on my webpages (link below) when I click on a thumbnail on my webpage to bring up the full-size pic. Try clicking on the first thumb on the page below (without scrolling down), then close the pic, scroll down the page and click on one of the set of six thumbs further down to see the problem.

Link

It comes up fine in IE, Google Chrome and Safari when I click on a thumb near the top of my page, i.e. the photo opens up with the whole screen behind it shaded in dark grey. When I scroll the page in IE, the dark background follows the image down all the way, as I would expect, but in Chrome and Safari, the grey shading stops at about 1000 pixels down and the undimmed webpage shows behind the full-size image.

I find this glitch really visually distracting and the white photo border melting into the main webpage background doesn’t look good – has anyone else encountered this problem and found a solution? Many thanks – John.