I think that you should be looking into your editor's settings.

I'm not really sure, but could it be your system cache? try disabling all the plugins that you installed after K2

I am using JCE editor. As a test, when I added a image to the k2 article, I changed the default image URL from "images/BaiYun/xxx.jpg" to "/images/BaiYun/xxx.jpg", then it can display in front end. But this is weird: very time I add a image, I need to add a "/" in front of the default image path. Also, in front end, it displayed message: K2_YOU_NEED_TO_LOGIN_FIRST

I also checked the JCE settings, the "File System Folder Path" is: images.

Don't know where is the problem: K2 or JCE or Joomla?