Create the backgrounds for the game, or work with the other background artist to end up developing it. 

For reference as to what style of art the game uses for its backgrounds, look towards Them's Fighting Herds, and mainly Skullgirls: 2nd Encore - the painterly, expressive background art is a key example of what kind of background work should be put inside the game - Something really important to note, separate as many parts of your background into different layers as possible, as all stages in the game actually use a 3D map, with the art textures on top of them to make a papercraft effect. 

For more, contact me on Discord - handle: baahorns