<div id="divpic" class="portlet" style="position: absolute; width: 600px; left: 50%; margin-left: -300px; z-index: 999; top: 788.273px; display: block;">
<img id="imgdiffrent" src="http://www.mamicode.com/20151015/164401248735" >
<div id="imgdiffrentdiv" style="color:blue;font-size:30px;position:absolute;width:600px;height:200px;z-indenx:9999;left:10px;top:220px;transform:rotate(-45deg)">
<b>仅供MyJobSky参考使用</b></div>
</div>

一定父层div  和子层 div position:absolute        z-indenx:9999 在最上层  -1 为最下层  

图片加水印效果