Hans, thanks for the response. I followed your recommendation and rescaled the image in the WP media library to 1920×1440, removed the old image from the Integral options page, uploaded the new one… well, it fixed the problem. Somehow, picking the same image but with a bigger size, flipped it upside/down.
On a separate note, is it possible with Integral Lite to use a background video instead of an image (e.g. what Airbnb, Paypal do on their sites)?