Edit: I defer to Keltex's answer. It's a much better solution. I'll leave mine here for posterity (unless I should delete the content and my answer entirely? I'm new here).
Another solution, which was used fairly frequently in the past, is to create a landing page that preloads all of your images. When the preloading is done, it redirects to the actual site. In order for this to work, you'd need to get the URLs to all of the images you want to load, and then do something like this:
# on index.html, our preloader
<script type='text/javascript'>
// add all of your image paths to this array
var images = [
'/images/image1.png',
'/images/image2.png',
'/images/image3.png'
];
for(var i in images) {
var img = images[i];
var e = document.createElement('img');
// this will trigger your browser loading the image (and caching it)
e.src = img;
}
// once we get here, we are pretty much done, so redirect to the actual page
window.location = '/home.html';
</script>
<body>
<h1>Loading....</h1>
<img src="loading.gif"/>
</body>