CDN for relative path Image

I have activated super cache and CDN support, and all seems working. JS and CSS files are correctly switched to CDN url.

However my practice for posts is to store image in separate folder (e.g img) and use relative path when using it with IMG tag. I have add “wp-content,img” in “Include directories” field but this seems not trigger switch to CDN if I check in HTML source, as it still shows relative “img/xxx.jpg”.

Is there a way to make this happen?

(if I spesificly write full url for IMG tag (e.g http://domain/img/xxx.jpg), CDN switch is working)

http://ww.wp.xz.cn/extend/plugins/wp-super-cache/