CSS3 实现图片上浮动画

.gist {
    width:220px;
    height:130px;
    background-image: url(2.jpg);
    background-repeat:no-repeat;
    border:5px solid green;
    background-position:50% 50%;
    transition:background-position 2s;
    -moz-transition:background-position 2s; /* Firefox 4 */
    -webkit-transition:background-position 2s; /* Safari and Chrome */
    -o-transition:background-position 2s; /* Opera */
}

.gist:hover {
  background-position: 50% -100px;
}
<div class="gist"></div>

图片水平垂直居中:
background-position:50% 50%;