The following help guide will show you how to add a coupon image at checkout to try and get customers to go back to your website and place another order in the future.
Step 1 - from your dashboard go to Orders and Coupons as shown below
Step 2 - Either edit an existing Coupon or add a new one in.
Step 3 - Browse for the Coupon image you wish to show at checkout.
Step 4 - If you wish to add some text to show along with the image enter it into the Coupon details box. Then tick 'Show On Confirmation Page' as shown below.
Step 5 - Your customer will now see this offer once they have placed the order.