The checkout page is controlled by Weebly so you can’t access it to change the background. Because of this, you can only change the background image for the checkout page via theme codes and this tutorial will show you how.
1) The image file for the checkout page background is light-header.jpg. First save the image you want to replace as light-header, and make sure the image format is .jpg
3) Click into “design” and into “edit html/css”
4) On the left hand column scroll to the “add new file(s)” section and insert your new light-header.jpg
5) Save your change and re-publish your site