The gift card template is the page someone who receives a gift card from your store. The page that sells gift cards looks just like every other product page on your store.
You can upload your own gift card image to replace the default Shopify image. In the theme editor, when on the Gift card template, find the "Gift card image" section in the left sidebar. Click into it and you'll see the button to upload your own gift card image. The recommended size is 800px wide by 533px tall.
Note that by adding your own image here, it does not automatically change the image that is in the original email sent to the gift card recipient. For now, we recommend you have a developer to do this. We'll soon be adding documentation that tells you how to do this step by step. In the meantime, you can reach out to us for instructions.
Here's an example of the default gift card template: