I know how to reference it from wikis but where in GitHub site do I upload the attachment? Thanks a lot!
As of Feburary 14th, 2022 GitHub allows uploading images to Wiki pages by dragging files into the web editor.
Caveat: Files will be uploaded to their githubusercontent bucket, not added to the git repository for the wiki.
Original post:
Just to answer it shortly: You can't. You have to upload your images somewhere else (like in a repository or on an image hoster) and link it
But I agree with @jstephenson