Can I add a custom background to a mockup?

The mockup tool gives you a neutral background and a transparent version of every shot. For a custom scene, take the transparent PNG into any image editor (or your ad/design tool) and drop it onto whatever background you want — a desk, a color, a photo.

If you're showing a series, the tool also lets you add the other books' front covers to a shelf-style shot — see Can I create mockups for a series?