Let’s just agree at this point that Github repos don’t generally have “default” images. They have userpics, and userpics are a terrible depiction of the repo’s content. Especially when stretched way beyond its normal size to serve as preview image.
You see, if you don’t have a natural image associated with your page, IT IS OK not to use any image at all. Really. Your engagement will be fine. You will be fine. And I will be happy.