That’s still going to be included in a standard clone, though at least it makes it possible to do a partial clone and exclude that branch. rjzzleep is right to point out the wiki repository as a good place to store such things, still being a Git repository so that it need not be tied to GitHub, but there’s also the option these days of editing the README in GitHub’s web interface and uploading the images or videos through that, which stores it on GitHub servers outside the repository.