I use the imagesLoaded javascript library to determine when and what images are loaded and to provide a status on a pre-loader status screen. The imagesLoaded library has the ability to gather information on background images, and non-background images using the 'background: true/false' argument. If you need to get both background and non-background images, just run it twice switching the background arguments. I highly recommend using this library when you want to provide a 'please wait' dialog and show the load progress of a graphically intense site.

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1// Get the parallax images (non-background images).
2$('.images').imagesLoaded({
3    background: false
4}).progress( function( instance, image ) {
5    loadProgress();
6});
7
8// Get the background images in the scenes.
9$('.bcg').imagesLoaded({
10    background: true
11}).progress( function( instance, image ) {
12    loadProgress();
13});