If preload is unchecked, the images are only loaded when they are dynamically entered into the DOM, which is the exact moment when the scene is opened. So on slow connections you will see a top-to-bottom loading motion for most images ( and a fade from blurry for progressive JPEGS ), whereas you would see images instantly if the images were preloaded.
One way to get the best of both worlds is include a brief buffer period before showing the images.
Or, if you're on scene 1 and you want to preload images from scene 2, you could keep the images from scene 2 off the visible scene area of scene 1. While the user won't see them on scene 1, when they switch to scene 2 they'll be ready to go in the browser's memory. Of course, this has implications for how scene 1 performs.
Hope that helps clarify!