html,body{ width:100%; height:100%; padding:0; margin:0;} 
#bcg {width:100vw; min-width:1120px;height:100vh; background-image:url(http://trabajando.unpocodetodo.info/images/mc1.JPG);background-size: cover; position:relative}
#draggable{ 
  width:400px; 
  height:400px; 
  position:absolute; 
  top:0; left:0; 
  border-radius:50%; 
  pointer-events:none;
  background-color:#abcdef;
  mix-blend-mode: difference;
}