Learn how to change background images on scroll with CSS.
Read on how to do it in this link: https://www.w3schools.com/howto/howto_css_bg_change_scroll.asp
Change Background Image on Scroll
Scroll down inside the frame to see the effect:
Try it yourself here: https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_bg_change_scroll