backgroundを使わない
<div>yyy</div>
<img class="wallpaper" src="https://i.imgur.com/Ye7Rs6O.jpg"></img>
<div>xyz</div>
img.wallpaper{
position:fixed;
object-fit:cover;
object-position:center;
width:110vw;height:110vh;
filter: blur(15px);
overflow:hidden;
top:-15px;
left:-15px;
transform:scale(1.2);
z-index:-100;
}