Any GitHub wiki can be cloned by appending wiki.git to the repo url, so the clone url for
the repo https://myorg/myrepo/ is: git@github.com/myorg/myrepo.wiki.git (for ssh) or https://github.com/my/myrepo.wiki.git (for https).
You make edits, and commit and push your changes, like any normal repo.
You need to clone the wiki repo and edit it on your system.
Add images to an images directory (or subdirectory below it) in your wiki repo. For example: images/project-architecture/project-architecture-overview.png.
In your wiki page markup, embed an image link in the following format:
[[/images/path/to/image.ext|ALT TEXT]]
Note the leading / in the path. You don't need it if all your wiki docs and image files are located at the top level of the wiki (like when you use the online wiki editor), but if you are working with a clone of the wiki, then you can organize files in subdirectories; and in that case, absolute or relative path specifiers are critical so that the path to the image resolves correctly when editing a doc that is located in a different subdirectory.
Commit and push your changes.
This is not the same as providing a link to an image that is part of your project repo (not the wiki repo). If you want to link to an image in your project repo, you will need a full URL that looks something like this:
[[https://github.com/username/repository/blob/master/img/octocat.png|alt=octocat]]
See references, below.
You need to clone the wiki repo and edit it on your system.
Add files to the files directory (or subdirectory below it). For example: files/project-presentation.pdf`.
In your wiki page markup, add the file link in the following format:
[link text](files/path/to/file "ALT TEXT")
For example:
[Project Presentation](files/project-presentation.pdf "Project Presentation PDF")
Commit and push your changes.