Wikis can display PNG, JPEG, or GIF images
Now you can use:
[[https://github.com/username/repository/blob/master/img/octocat.png|alt=octocat]]
-OR-
Follow these steps:
On GitHub, navigate to the main page of the repository.
Under your repository name, click Wiki.
Using the wiki sidebar, navigate to the page you want to change, and then click Edit.
On the wiki toolbar, click Image.
In the "Insert Image" dialog box, type the image URL and the alt text (which is used by search engines and screen readers).
Click OK.
Refer Docs.