.contenedor {
  position: relative;
  float: left;
  margin-bottom: 40px;
}
 @keyframes anim{
  0% {opacity: 0;} 
   25% {opacity: 0.5;} 
   50% {opacity: 1;} 
   75% {opacity: 0.5;} 
  100% {opacity: 0;} 

}
.contenedor img {
  position: absolute;
  left: 0;
}

 img.top {
   animation-name: anim;
  animation-duration: 3s;
  animation-iteration-count: infinite;
}