structuring media uploads (users, images, videos) using sequelize.js
I am leaning towards Option 1 for this because I actually think that as you add more media types option 2 is going to get unmanageable when it comes to querying, especially when you are trying to get all of a user's media, it would just require too much eager loading of the other tables, or a lot of join tables. 

With option one, each piece of media has it's own unique id, so if you set up the association between Users, Media, and Likes so that:
Likes belongTo users 
Users can hasMany Likes 
Media can hasMany Likes 
Likes hasOne media (so each individual like is a representation of it's userId and the mediaId it was associated to) 

then you would be able to query your Likes table for all Likes with a certain userId (to show all the media a user has liked), or for all Likes for a certain mediaId (ex. to show the amount of likes for a specific image). 

You wouldn't even necessarily have to specify the media type when querying likes because you'd have your mediaId or UserId depending on what you are trying to query.