建站常识

请问用js怎样把这两张图做成动画，走一段距离后，沿原路线返回

<style>
.an{
    width: 155px;
    height: 88px;
    animation: anim 0.6s infinite;
}
@keyframes anim{
    from{background-image:url("1.png") ;}
    to{background-image:url("2.png") ;}
}
</style>

<div class="an">	
</div>