#overlay {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: #fff;
  z-index: 9999;
}

#center-image {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  z-index: 1;
  width: 70%;
}
.pc{ display: block;}
.sp{ display: none;}
@media screen and (max-width: 768px){
  .pc{ display: none;}
  .sp{ display: block;}
  #center-image {
    width: 94%;
  }
}