An alternative might be to create a simlink in a directory already readable by joplin-content://.

To do this:

  1. Determine the path to Joplin's cache directory.
    • For me, it's /home/self/.config/joplindev-desktop/cache
  2. Create a subfolder that simlinks to /home/user/Pictures
  3. Use joplin-content://note-viewer/<cache directory path>/<simlink name>/photo.png for photo URIs.