This used to work in Picard 1.2, but broke during the cover art refactoring.
This is the commit that broke it:
The old code in file.py used this to evaluate the script:
image_filename = self._script_to_filename(image_filename, metadata)
In the current code the metadata is still passed to Image._make_image_filename but it is not used there. Should be easily fixable, but requires some refactoring to not duplicate code from file.py in image.py