当整个类传递给背景值时，这是什么语法

Question

class Gallery extends React.Component {
  render() {
    const imagesLoadedOptions = { background: '.my-bg-image-el' }

    return (
        <Masonry
            className={'my-gallery-class'}
            elementType={'ul'}
            options={masonryOptions}
            imagesLoadedOptions={imagesLoadedOptions}
        >
            <div className="my-bg-image-el"></div>
        </Masonry>
    );
  }
}