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
The following markdown will reference an image in your GitHub repo from within your corresponding GitHub wiki

The above example assumes that you have the following file structure in your repo
For a real world example, if I wanted to reference this image in github from the corresponding wiki, I'd use this markdown

Caveats
For Home Page:
