README.md: use static okegui cover image

Author: sinsanction

README.md

@@ -1,7 +1,7 @@
 # [OKEGui](https://github.com/vcb-s/OKEGui/) · [![GitHub license](https://img.shields.io/badge/license-GPLv2-blue.svg)](https://github.com/vcb-s/OKEGui/blob/master/LICENSE) [![Build status](https://ci.appveyor.com/api/projects/status/p4p7upa6hmsgu599?svg=true&passingText=%E7%BC%96%E8%AF%91%20-%20%E7%A8%B3%20&pendingText=%E5%B0%8F%E5%9C%9F%E8%B1%86%E7%82%B8%E4%BA%86%20&failingText=%E6%88%91%E6%84%9F%E8%A7%89%E5%8D%9C%E8%A1%8C%20)](https://ci.appveyor.com/project/vcb-s/okegui)
 
 
-![alt text](http://www.ajivin.com/images/spsimpleportfolio/site-clearing/portfolio6_600x400.jpg)
+![alt text](okegui_600x400.jpg)
 
 ## 安装与使用