const setPainting = painting => {
  return {
    type: 'GET_PAINTING_SUCCESS',
    painting
  }
}

...

export const fetchRandomPainting = () => {
  return dispatch => {
    return fetch(`api/random`)
      .then(res => res.json())
      .then(painting => dispatch(setPainting(painting)))
      .catch(error => console.log(error));
  }
}