How to add a background image for the page header section (left box)
The original design for the regular page template is to automatically add the very first image of the page as the title background (on the left side container) - demo example.
So all you have to do is to upload an image before any other page content. If you have any text or empty paragraph on the body section (displayed in the right container) all the page content will display in the page content section, with no background for the title page.
This is the only way to display a background image here because Shopify does not allow custom fields on each page.
In case the image is not displaying correctly as background for the page header section, please make sure that the image is included in a paragraph tag as in this screenshot.