Divh

 /* Desktop Thumbnail */

.thumb-desktop {

  display: block;

  width: 100%;

}

/* Mobile Thumbnail */

.thumb-mobile {

  display: none;

  width: 100%;

}

/* Mobile Screen */

@media screen and (max-width: 768px) {

  .thumb-desktop {

    display: none;

  }

  .thumb-mobile {

    display: block;

  }

}

<img src="DESKTOP_IMAGE_URL" class="thumb-desktop" />

<img src="MOBILE_IMAGE_URL" class="thumb-mobile" />