I see that the images are setup with opacity.
This could be done in a custom page template, extract the image names relating to that page, put those names into inline css code.
Most of the css and scroll mechanism etc can be traditional and static, picking up image file names out of page content is something I have not previously seen, but will certainly work.
Your pages may look like:
"image1 name","image2 name",conventional page contents
In the page template, you will fetch the page data from conventional WordPress, strip off the leading image names, keep the conventional page contents.
So you need to get your theme sorted out with their adjusted opacity, and then you have to code your custom page templates.
Then you will be able to code your pages with the image names in the page contents, which is your goal.