With upcoming paste improvements, there will be more types of sources that the image block should be able to handle.

• data: (base64 encoded)
• file: (local files) Not possible sorry. They are now deleted in c9e6e62, but a placeholder is kept. We could also inform the user that they need to re-add them.
• External images. We should not hot link to e.g. a Google CDN. The image should be copied to the server.