Simply uncheck the box to turn on the custom Fordson Login page here: Site Admin > Appearance > Fordson > Custom Login .
Use the default login page.
Upload your own background image on Fordson > Custom Images.
Seeing the screenshot makes me think you wanted to use the custom login page for Fordson. The custom login page has a look to it that is built in. However, you can use the Moodle default login which would give you a full screen login image with the login box centered.