Thanks for asking - this is actually the subject of a pending update to the download edition, as there's a much easier alternative method!
1. Set up an account at imgur.com
2. Upload your image
3. Select it and copy the Direct Link URL listed on the right. This produces something like https://i.imgur.com/kK9mjbp.png , which you can use in your game.
WARNING: Google Drive links only work for a limited time (two weeks, I believe)
1. Upload the image file to your Google Drive
2. Right-click it in your browser and select Share
3. Set it to Public on the web (Anyone on the Internet can find and view)
4. Visit the sharing link (in this case https://drive.google.com/file/d/0B8rBdbbzY_94NlBjNEJqLVNsWlE/view?usp=sharing)
5. Right-click on the image and select Copy image address to get the direct link - in this instance that's https://lh3.googleusercontent.com/I7uYFAIdCN-rpB4JCAo_K2bwY8Y6QcwfnxZImNI6usMrJ0... <-- functional but curtailed by editor
N.B. This direct link will stop working after two weeks so is only suitable for short-term projects.
The update should be going into the Download Edition code and comments by Wednesday. I hope this helps. Thank you for the reminder!