url: http://placehold.it/
http://placehold.it/350x150.jpg/aaa/fff
[http://placehold.it/width×height.(png,jpg,jpeg,gif)/background color/text color]
http://placehold.it/350x150.jpg/aaa/fff?text=aaa+bbb
[add text content]
原文：http://www.cnblogs.com/xjnotxj/p/5325579.html