Which page are you wanting to add the background image too? By the looks of things, you’ll need to do this by altering the liquid code. This customization isn’t supported by Shopify, however, a Shopify Expert would be able to do this for you. Shopify Experts are trusted, third-party agencies and freelancers who offer services for Shopify merchants. To browse Experts, or to post a job, head on over to the Shopify Experts website. Here you will connect with Shopify Experts who you can hire for complicated or difficult tasks related to your business, such as customizing your theme. When you contact a Shopify Expert, you’re not committed to hiring them, you decide whether you want to work with them.