Thank you for reaching out and posting this question!
I understand you're looking to replace the background image for the password page on the Debut theme. Since you're using a Shopify-supported theme, this is something that our Theme Support team can help you with. If your store is subscribed to plan, then you're eligible to use the complimentary design time included with your subscription on this customization. While we’re not able to provide account-specific support via the Shopify Community at this time, our Support team would be happy to assist you with a customization request through live chat, email, or callback. Please visit the Shopify Help Center and log in to your account to create a support request.
Also, would removing the "Powered by Shopify" message from your footer be something that interests you?