IE6: Background-Image Load Event

I am displaying a bunch of thumbnail images and the latency can be very high (over a VPN) so I send all the thumbnails in a single file (like a sprite) and set the CSS background-image and background-position properties of a div to show the thumbnails. The problem I'm having is with IE6 and figuring out when the image has loaded... I'm using the BackgroundImageCache hack:

Firefox and Chrome stops displaying my sprite after 655 images

the problem

Hi, i'm working with sprites and got some interesting behavior of firefox and Chrome. When my sprite have 655 images or less, my sprite works fine. But when it goes furter (656 or more) it doesn't get displayed (just become invisible). I'm guessing the problem is not in my code since it works fine in Opera and IE. How come?