So you need to store images per each state and associate them with states somehow and I think multiple options to implement it.
For example create page type (State-Banner) that will hold an image and state name and/or abbreviation and create appropriate page for each state and place them all into some folder in content tree, so editors could navigate it in content tree and update through Form tab.
Or you could name images with state name or abbreviation and upload them to media library again into some folder dedicated to banners.
I believe there are other options as well.
Also in both suggested approaches I'd recommend to keep default banner in case some state get 'lost' or someone unintentionally removes an image.