.sheet-background {
   width: 800px;
   height: 450px;
   z-index: 0;
   position:absolute;   

}

.sheet-background::after {
   content: "";
   top: 0;
   left: 0;
   bottom: 0;
   right: 0;
   position: absolute;
   z-index: -1;   
   background: url('http://i.imgur.com/4OKDPud.jpg');
   background-size: 800px 450px;
}