An alternative might be to create a simlink in a directory already readable by joplin-content://.
To do this:
- Determine the path to Joplin's cache directory.
- For me, it's
/home/self/.config/joplindev-desktop/cache
- Create a subfolder that simlinks to
/home/user/Pictures
- Use
joplin-content://note-viewer/<cache directory path>/<simlink name>/photo.png for photo URIs.