@media only screen and (max-width: 640px){
#page .page-section:nth-child(1) .section-background img {opacity:0 }
#page .page-section:nth-child(1)  .section-background {
background-image: url(your-image-here);
background-size: cover;
background-position: center;
background-repeat: no-repeat;
}
}