How do I replace background video with fallback image on mobile?

Mobile Breakpoints

/* CSS FOR TABLET AND MOBILE */
@mobile:  ~"only screen and (max-width: 640px)";
@tablet:  ~"only screen and (min-width: 641px) and (max-width: 949px)";

/* CSS FOR TABLET */
@media @tablet {
/* Insert Code for Tablet Below This Line*/

/* Insert Code for Tablet Above This Line */
}

/* CSS FOR MOBILE */
@media @mobile {
/* Insert Code for Mobile Below This Line*/

/* Insert Code for Mobile Above This Line */
}

Mobile CSS

#PAGEURL img {
  display: none;
}

#PAGEURL .sqs-block-image img {
  display: block;
}

#PAGEURL {
  background: url('IMAGEURL');
  background-repeat: no-repeat;
  background-size: 100%;
}