I want to display a different background image on mobile for every page

Hi

I'm desperately trying to change the background image for mobile as the desktop version makes the copy that overlays it difficult to read. I need to do this for every page of the website (5 in total). 

I've read many different posts and watched a lot of different YouTube videos. None of the CSS I've tried is working. 

For example I've tried this code, which doesn't work:
 

@media only screen and (max-width: 780px) {#collection-64f6f5c69386bb2b883244fb {

#page .page-section:nth-of-type(1) {

img {

display: none;

}

.sqs-block-image img {

display: block;

}

.section-background {

background: url("https://images.squarespace-cdn.com/content/64f6f4b8ca44186f5f04420c/19333f4d-a055-4992-ae2a-14cf7f7c9697/home-mobilehero-image.jpg?content-type=image%2Fjpeg");

background-repeat: no-repeat;

background-size: 100%;

}

}

  }}

When I click save, and view it on mobile nothing happens.

Could anyone help please?

Many thanks

@Waggit Would it be possible to share your site URL so I can take a look? Thanks!