Yes, you can replace your gift card product images. Not just that - you can also update other details such as the gift card title, validity date, and the prefix number for the cards sold online.
To update the gift card template displayed on Webstore:
From the organization level, navigate to Admin > Setup > Predefined Gift Cards and select the template that you want to update.
Based on what you want to update, click the appropriate tab:
General: Update the gift card details
Centers: Make the template available or remove it for specific centers
Catalog: Upload or browse for an image.