Applying a unique image or color background to specific pages can get a bit tricky, involving custom code changes that are specific to your theme, version, page handles, image URLs, color selections, etc.
For this type of specific customization we suggest reaching out to one of our recommended Shopify Experts, with whom you can consult to bring your customization visions to life!
To check out your default background settings, from your Shopify Admin, head into Online Store > Themes > Customize > Theme Settings > Colors, where you will find settings to apply background colors to your online store.