Promo cards are used to promote the brand or particular initiatives such as promotions that you undertaking.
For editing images to fit perfectly within the promotional card images frame in your app, you need to balance the width vs file size. Most phones are just over 1000px wide so around 1000px will render well, but if it's very detailed, and you will want a PNG format and you might want to push it down to 800px to keep file size down.
For many clients 1080x607px works well.