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