.pk-fade-slider {
  position: relative;
  width: 100%;
  overflow: hidden;
}

.pk-fade-slider img {
  position: absolute;
  top: 0;
  left: 0;

  width: 100%;
  height: auto;

  opacity: 0;
  animation: pkFade 21s infinite;
}

/* 1枚目だけ高さを確保するためrelativeにする */
.pk-fade-slider img:first-child {
  position: relative;
}

.pk-fade-slider img:nth-child(1) { animation-delay: 0s; }
.pk-fade-slider img:nth-child(2) { animation-delay: 3s; }
.pk-fade-slider img:nth-child(3) { animation-delay: 6s; }
.pk-fade-slider img:nth-child(4) { animation-delay: 9s; }
.pk-fade-slider img:nth-child(5) { animation-delay: 12s; }
.pk-fade-slider img:nth-child(6) { animation-delay: 15s; }
.pk-fade-slider img:nth-child(7) { animation-delay: 18s; }

@keyframes pkFade {
  0% {
    opacity: 0;
  }

  3% {
    opacity: 1;
  }

  14.28% {
    opacity: 1;
  }

  17.28% {
    opacity: 0;
  }

  100% {
    opacity: 0;
  }
}