.bt1{
    width: 640px;
    height:58px;
  position: relative;
  animation-name: example;
  animation-duration: 4s;
}
.bg {
  width: 100%;
  height: 100%;
  position: absolute;
  z-index: -11;
  top: 0;
  margin: 0 auto;
  background-color: transparent;
  background-image: url(images/bg.jpg);
  background-position: center;
  background-repeat: repeat;
  background-size: cover;
}

@keyframes example {
  from {width: 640px;height:58px;}
  to {width: 660px;height:60px;}
}