.testbg {
	background: url('https://picsum.photos/1920/1080/?random') no-repeat fixed;
	width: 100%;
	height: 100%;
	z-index: 1;
	top: 0;
	left: 0;
	position: fixed;
}
@media screen and (max-width: 1080px) {
	.testbg {
		background: url('https://picsum.photos/1080/1920/?random') no-repeat fixed;
	}
}