For what it's worth, I did something similar to the Apple site for a client. I tried two approaches, both using individual img elements (not a sprite) positioned fixed behind the normal content.   The first I tried to create animations and ScrollTriggers for each section of the page and then sync those to the correct timing with the background images. While this worked (and was easier to get pinning working), it proved to be quite difficult to keep things synced on different viewports.
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