Sorry, I was not replying to your post but to the previous one that suggested to use shortcodes, which are not makdown.
Regarding your post, I totally agree that there should be a compatibility with github links. Nevertheless, I think that the issue is more general : a markdown document should not define image paths because they depend on the rendering context. Image paths in github and website or while rendering PDF for instance will be different.
Thus I think that there should be a way to tell Hugo the path for images and it would set it correctly for various document locations.
I have fixed this issue using pandoc with a filter that sets the path of the image while processing.