<img class='img1' src="https://www.bbvaopenmind.com/wp-content/uploads/2015/05/pexels-photo-462118-1.jpeg" alt="butterfly" />

<img class='img2' src="http://4.bp.blogspot.com/_EZ16vWYvHHg/TFBW_zN6oaI/AAAAAAAAQd4/8UxrcXLQ5js/s1600/www.BancodeImagenesGratuitas.com-Perrito.jpg" alt="dog" />

body{
  background-color: #ccc;  
}

img{
  display: block;
  margin: 0 auto;
  margin-top: 10px;
  border: 2px solid black;
}
.img1{
  width: 600px;
  height: 300px;
  animation: animateImg 2s linear alternate infinite;
}

.img2{
  width: 500px;
  height: 300px;
  animation: animateImg 2s linear -2s alternate infinite;
}

@keyframes animateImg{
  from{
    filter: blur(0px);
  }
  to{
    filter: blur(3px);
  }
}