The Github wikis are backed by a separate git repository then the main project repository.
This post by the Github team makes it sound like you should be able to link to
Relative path work for me like this:
home page of the wiki:
![text](wiki/images/someimage.png)
sub page of the wiki:
![text](images/someimage.png)
Note that when doing a preview the image will not show, I had save it.