I am using GitHub Pages, Jekyll, and Prose to manage a Website. Everything works fine, besides I cannot find a way to upload images. The Prose image uploader is not working
You can also use GitHub Desktop. Then just add the repo, add images to the folder from your computer, commit and sync. This is essentially the command line git functionality but in the form of a GUI, making it easy for your friend to use.
P.S. Yes, I know this isn't actually on the Github website but it would solve your issue of giving your friend a way to upload images easily from Github.