Tiles not showing, background shows white.

Hey all, my blog is at chrisfawkes.net

You will see i have a tiled background. I have also set a background color so if the tiles do not show for any reason the color scheme is still pretty close.

I checked at an internet cafe and the background just shows white. I went to the apple store and same thing on their new Macs.

Is there some code i can add in to fix this, is it likely to just be related to internet cafes etc

Any advice appreciated.

Cheers.