Fork the repo. Clone a copy of your fork on your local Create a branch off of master and give it a meaningful name (e.g. my-new-mediumish-feature). Make necessary changes, commit, push and open a pull ...

If you want to use the update script, copy an arbitrary number of images you would like to use to minegrub/backgrounds/. You can find some options in background_options/ but you can also use your own ...