I\'m trying to figure out a good structure to create a many to many relationship in Redux. For this example we have images (which can have multiple tags) and tags (which can
Yep, this is an example of a "normalized" state. The Redux docs cover normalization in the Structuring Reducers section. See Structuring Reducers - Prerequisite Concepts, Structuring Reducers - Normalizing State Shape, and Structuring Reducers - Updating Normalized Data. Also, the Redux FAQ discusses normalization in "How do I organize nested or duplicate data in my state?".
Beyond that, I'm a big fan of using a library called Redux-ORM to manage that relational data in my Redux store. I've written a couple posts that discuss how I use it: Practical Redux Part 1: Redux-ORM Basics and Practical Redux Part 2: Redux-ORM Concepts and Techniques. The rest of my "Practical Redux" tutorial series also shows this library in action.