jQuery image preloader function, pass an url/id and it will run the callback once the image has loaded. Natch.

jquery_image_preloader.js

preload_image : function(image, callback) {
  $preload_images = $('#preload-images');
  if($preload_images.size() < 1) 
  $preload_images = $('<div id="preload-images" style="display:none" />').prependTo('body');
  $preload_images.append('<img data-id="' + image.id + '"src="' + image.src + '"  />');
  $preload_images.find(' img[data-id=' + image.id + ']').load(function(e){
    if(typeof(callback) == 'function') callback();
  });
}