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
If you want to quickly upload an image with drag/drop, you can perform the following (albeit hackish):
Create a dummy issue; drag & drop your image there; copy/paste the uploaded markdown image code to your wiki;
After you create the issue once, you can use it any number of times to do this.
Hope this helps anyone looking for a quick fix, without needing to have the image reside in the repo.