In practice a second or two at the most in this case as they are only 60-100k files.
But it's more to do with how excellently 'snappy' a site feels when the next image is preloaded, along with how crappy it feels if you have to watch it paint. Try http://www.mastermaster.org.uk/new3.php and check the dfference between flicking through really fast (into uncached images!) and waiting for the next one to preload.
If you don't preload, your connection speed becomes [a negative] part of the presentation experience. Whereas presentation should be as far divorced from the delivery medium as possible, I feel. I haven't explained that well but I expect YSWIM.
I reckon if the site developer knows what image is coming next it's sloppy not to preload it.