Should I use additional id values as unique values that only applies in junction tables in MySQL? I have created three tables called person, metadata_image and person_image.

person table contains info about people.
person_id
person_name
person_bio

metadata_image table stores the images.
image_id
image_url
image_type
image_size
date_uploaded
importance (if importance equals to 1, I use that image as cover picture)

person_image has foreign keys from person and image tables.
person_id
metadata_image_id


Additional image : 



Should I add a third column like person_image ID as a unique and auto increment value or is my structure okay as it is?
With just the two foreign keys in there I would leave it as it is.

If other tables have a foreign key pointing to person_image then the debate becomes a lot more nuanced and you will start considering table size and query (plan) complexity.