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 images stored in the wiki repository inside the wiki markup.
Images and Folders
You can now reference images hosted inside the Git repository.
I've checked out my wiki's repository and pushed a resource folder and an image into it. My question is, how do I go about using this image?
Is this even possible or have I misunderstood something?