Upload image coupons at 400 by 200 pixels so they display correctly in the standard coupon view.
That 2:1 ratio matches the default coupon frame, so a 400×200 image fills the space cleanly without stretching or leaving gaps. You can go larger as long as you keep the same proportions (for example 800×400) and let the coupon scale down to fit its container; very large files just take longer to load.
If you use Coupon Creator Pro, the image should match the size you set for the Print Template instead, since Pro lets you control the coupon dimensions for both the on-page view and the print view.
For a step-by-step on building one, see Creating an Image Coupon.