#parent {
   position: relative;
}
img.superpose {
   position: absolute;

}
#img_1 {
   z-index: 10;
   left: 415px;
   height: 400px;
   top: 290px;
   width: 500px;
}
#img_2 {
   z-index: 11;
   left: 280px;
   top: 290px;

}
#img_3 {
   z-index: 12;
   left: 555px;
   top: 510px;
   width: 200px;

}

#img_4 {
   z-index: 9;
   left: 360px;
   top: -260px;
   height: 600px;


}

html
{
    background: url(https://www.informatiquegifs.com/nosgifs/feu-artifice/feu-artifice.gif) no-repeat center center fixed; 
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;

}