.back01 {
  position: fixed;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 400%;
  background: url(../images/bg_petal01.png) center top;
  -webkit-transition: -webkit-transform 5s cubic-bezier(0.19, 1, 0.22, 1);
          transition: transform 5s cubic-bezier(0.19, 1, 0.22, 1);
}

.back02 {
  position: fixed;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 400%;
  background: url(../images/bg_petal02.png) center top;
  -webkit-transition: -webkit-transform 8s cubic-bezier(0.19, 1, 0.22, 1);
          transition: transform 8s cubic-bezier(0.19, 1, 0.22, 1);
}