Thank you for reaching out to the community forums with your question! I understand you want to add a custom image to the checkout page to help with your customer conversion and that you want to be able to do so in a way that is easy for you to manage.
The access to the checkout is extremely limited in order to help protect the privacy of your store and your customers. You do have the ability to add a logo, banner and background image to the checkout, but you will not be able to access the code to input your own custom embedded image.
I would recommend adding that image maybe to your cart page or on your shipping policies page instead.