Thanks for the feedback!
While the file:// protocol isn’t supported, both relative and absolute paths are, and you have a few options.
If your markdown file has been saved to /home/nigel/django/sarah/silsondb/static/site/my_file.md, you could use:
For relative paths like this, you can also use autocomplete while typing the path (by typing Control+Space). This has some usage caveats: Your file must have been saved, and the images must exist somewhere inside your selected notes directory.
Lastly, you can drag and drop images directly on to the editor to create a markdown image with the path prefilled.
Hope that helps!