.pic {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  -webkit-animation: fade 20s infinite;
  animation: fade 20s infinite;
}
#pic3 {
  background-image: url('../img/dashboard.png');
  animation-delay: 10s;
}
#pic4 {
  background-image: url('../img/foo.png');
  animation-delay: 20s;
}
@-webkit-keyframes fade {
  0% {
    opacity: 1;
  }
  15% {
    opacity: 1;
  }
  25% {
    opacity: 0;
  }
  90% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}
@keyframes fade {
  0% {
    opacity: 1;
  }
  15% {
    opacity: 1;
  }
  25% {
    opacity: 0;
  }
  90% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}