Post and pages uses the featured image option to show an image as background in the hero unit. If the featured image is not set the default image is used. You can learn how to set the default image here.
When adding a new page, on the right hand side, there is a box labeled “Featured Image”. If this option is not available check your screen options at the top right. Click the link “set featured image” and add your image. For best result use an image which is at least 800 pixels width.
