Вы можете создать свой собственный компонент:
const CustomFooter = ({ isModal, currentView }) => isModal && (
<div className="react-images__footer">
<button
className="btn some-stylin"
type="button"
onClick={() => { downloadImage(currentView.src); }}
>
Download
</button>
</div>
);
<ModalGateway>
{modalIsOpen ? (
<Modal onClose={this.toggleGallery}>
<Carousel
views={this.imgs}
currentIndex={currentIndex}
frameProps={{ autoSize: 'height' }}
components={{ Footer: CustomFooter }}
/>
</Modal>
) : null}
</ModalGateway>