You can use an image for the background with some custom CSS. Just note that the main content background is set to none so the background image is visible.
- Sign into your Shopify admin
- Go to 'Online Store' > 'Theme'
- Click 'Actions' > 'Edit Code'
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Scroll down to the 'Assets' Folder and select the 'theme-custom.scss.liquid' file
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Paste the following line of code in:
body { background-image: url(<http://image-url>) }
If you want the background to affect the main content (not the header or footer), then paste in the following line of code in place of the one above:
#app-body { background-image: url(<http://image-url>) }