I forgot a critical piece of info... these are all INDEX PAGES.
So, what I tried didn't work!
But here's what I tried in PAGE HEADER CODE INJECTION:
@media screen and (max-width:900px) {
main.Main {
background-image: url(https://static1.squarespace.com/static/5dade9b43aae383ce86b5c91/t/5db72a9fb8949143ca9237d0/1572285095291/laura-thonne-8CJ6HSeCWU0-unsplash-614x1024-1.jpg), !important;
background-size: cover;
background-repeat: no-repeat;
background-position: center center;
}
}