html, body {
height: 100%;
margin: 0
}

@keyframes breath {
0%   { background-size: 50% auto; }
50% { background-size: 70% auto; }
100% { background-size: 50% auto; }
}

#bkg{
width: 100%;
height: 50%;
position: absolute;top: 30%;
animation: breath 4s linear infinite;
background: url("https://media.giphy.com/media/zBg3B1KnmX69i/giphy.gif") center center no-repeat;
}