ER-Diagram for Users, Images and Likes

I’m looking to create a database (using Django) where:

  • A user uploads photos.
  • A user upvotes/downvotes photos of other users.

I came up with this ER diagram: enter image description here

My questions are:

  • Is a vote better as an entity or as an attribute of the photo?
  • If a vote should remain an entity, how do I show in its attributes that the primary key of the vote is the combined foreign keys of the user and the photo?

Any constructive comments/recommendations are also welcome.