Missing images

Hi all I have kind of a very strange problem... But I hope someone can help. Imageing a little web-based application for browsing images. Every page is exactly the same (from a php point of view). A user can browse through all the images which are displayed as thumbnails. Normally, everything works fine... But on some browsers (and or operating systems), suddenly, at a certain point, all images are turning white. No more new images are shown. Old images (which have already been shown earlier) are fine, but no new ones. They are just white. Reloading the page or reentering the URL doesn't work. The only thing which does work is shutting down the browser and restarting the whole thing. So, why do some images turn white? These normally happens after about the 6th page of images (12 iamges a page). Any idea? Thanks a lot reto