chore(readme): replace screenshot

README.md

@@ -20,7 +20,7 @@ This is a [demo on Heroku](https://hexxa-quickshare.herokuapp.com/).
 
 (User is `demo`, password is `Quicksh@re`, uploading/downloading speed is limited to around 100KB.)
 
-![Quickshare on desktop](./docs/imgs/desktop_2.jpeg)
+![Quickshare on desktop](./docs/imgs/desktop_2.png)
 
 Choose Language: English | [简体中文](./docs/README_zh-cn.md)
 

docs/README_zh-cn.md

@@ -20,7 +20,7 @@
 
 (用户名是 `demo`, 密码是 `Quicksh@re`, 上传/下载速度被限制在大约100KB.)
 
-![Quickshare on desktop](./imgs/desktop_2.jpeg)
+![Quickshare on desktop](./imgs/desktop_2.png)
 
 选择语言: [English](../README.md) | 简体中文