Document how to best include images in annotations

[badboy]

Right now we have at least 2 instances (see #169) where we link to GitHub-hosted images (because that's the most immediate way if you just drag'n'drop an image into the GitHub editor).

We should:

• probably pull those images into the repository
  • afaik github image hosting is stable, but who knows when that will go away
• Document what to do with images
• Stretch: Serve them through our github pages (the one that also hosts the JSON itself)