#mock1 {
  position:absolute; /* exact centering */
  top:50%;
  left:50%;
  width:640px;
  height:420px;
  margin-left:-320px; /* half the width */
  margin-top:-210px; /* half the height */  
  z-index: 1;
  
}
#mock1bg{
  position:absolute;
  background-image:url(bg.png);
  background-repeat:no-repeat;
  background-position:top;
  top:50%;
  left:50%;
  width:880px;
  height:420px;
  margin-left:-440px; /* half the width */
  margin-top:-210px; /* half the height */  
  z-index: 2;
}