<div id="small_gal">
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
    <div><img src="images1.jpg" /></div>
</div>

这些图像有阴影,所以边框不是一个解决方案,因为它将在图像之外

边界已经过渡,它在FF上平稳运行,但不能在chrome或其他浏览器中运行

这是我使用的代码

#small_gal div:hover{cursor: pointer;}
#small_gal div:after {
    content: '';
    position: absolute;
    width: 112px;
    height: 81px;
    border: 3px solid #e27501;
    left: 9px; top: 9px;
    z-index: 9;

    opacity: 0;
    -webkit-transition: opacity 0.5s ease-in-out;
    -moz-transition: opacity 0.5s ease-in-out;
    -o-transition: opacity 0.5s ease-in-out;
    -ms-transition: opacity 0.5s ease-in-out;
    transition: opacity 0.5s ease-in-out;
}
#small_gal div:hover:after {
    opacity: 1;
}

注意：

#small_gal

只是容器
每个图像被包裹在一个div中,所以我们可以实现：after