As a project grows and changes it's a more sustainable way to manage and track changes by multiple users. Tracking assets in version control is a good thing.
The gh-pages branch has a particular behavior on GitHub which is not necessary for hosting CDN images. Another person working on the project maintaining the gh-page may not know something external depends on the path to these images. It's a pain to migrate this solution to a fork, or off GitHub.
Many of the posted solutions are incomplete or not to my taste.
