Image link not correct at index page when site is under subdirectory

[Tomyail]

hexo: 3.1.1
os: Darwin 14.4.0 darwin x64
http_parser: 2.3
node: 0.12.7
v8: 3.28.71.19
uv: 1.6.1
zlib: 1.2.8
modules: 14
openssl: 1.0.1p

I put a image under the source folder and use it like this in my post

![image](../image.jpg "title")

This works fine when I set the root to /

But when I set the root to /blog/,on the Index page I got the image link like

http://mydomain.com/image.jpg. so the link not exist.

And by the way,the image link is correct in the post's detail page