Update README.md

Author: Nagi-ovo

README.md

@@ -61,7 +61,7 @@ Issue、PR、纠错、资料、选课/考试攻略，完全欢迎！
 ### c. 下载全部文件
 
 <div align="center">
-    <img src="https://jsd.cdn.zzko.cn/gh/Nagi-ovo/picx-images-hosting@master/docs/bjut-helper/how2download.41y032kg6z.jpg" alt="instruct" width="500" height="500"> 
+    <img src="https://cdn.bjutswift.cn/https://raw.githubusercontent.com/bjut-swift/BJUT-Helper/master/images/how2download.jpg" alt="instruct" width="500" height="500"> 
 </div >
 
 1.点击蓝色的 Code 部分；