Use below code, I hope it will solve your problem. Still any issues, Please let me know
@media only screen and (max-width: 640px){
body:not(.view-item) #page .page-section:nth-child(1) .section-background img {opacity:0 }
body:not(.view-item) #page .page-section:nth-child(1) .section-background {
background-image: url(https://static1.squarespace.com/static/62243b39a0f8a94fd0f4b835/t/622a92407674cb7948261a63/1646957124561/Mobile-bkg.png);
background-size: cover;
background-position: center;
background-repeat: no-repeat;
}
}
If you see your page, that was any collection page and you want specifc page and section 1 background image. The mean it
