Creating a background or image that changes when button is selected

I can’t seem to create a background that changes with a button. I know I can do this with images that aren’t a background, but I can’t get those to sit behind and scale like an ‘Intro Header’ does. I am new to this, but I’ve exhausted tutorials looking for how to create this:

Here is my site Read-Only: LINK
(how to share your site Read-Only link)

CSS styles can be manipulated via JavaScript through the CSS Object Model. See CSS Object Model (CSSOM) - Web APIs | MDN

Since JQuery is running on all sites you could also use that to change the background image when a click event happens. jQuery css() Method

For a more complete example look at the solution provided here → html - Change css background-Image using javascript - Stack Overflow