Customize
Notecard backgrounds cover both the top and bottom of the interior of the folded card.
The template shows the possible placement of the QR Code as well as the 'safe edge' and 'center fold' to help align your design.
Delete Placeholders
Be sure to DELETE THE PLACEHOLDER GRAPHICS, you don't want those printed onto your card.
Download
Click 'Download' at the upper right, and then 'Download' again to save the file to your device.
Upload to Image Templates
In thanks.io, click on Image Templates on the left. Then, scroll down to the last section in the center of the page titled 'System Default Backgrounds.'
Click on the card size/type you want to upload your new background to.
When you build out or schedule mail in the future, the specified background will be set automatically.