Aligning the background image

I want to create a custom theme for my survey by uploading and including an image file. Among the themes available on Zoho, the beauty one is closest to what I want mine to look like. The image is displayed on the left side of the screen and the full survey is visible on the right side. I just want to be able to use my own image but achieve this same effect.

However, when I upload an image to create the custom theme, it underlays the survey so that the image is only partially displayed. No matter whether I check left, center, or right, the survey questions remain on top and partially obscure the image. How can I make it so that my custom image is displayed in the same way as the image used for the default beauty theme?

Thank you for any help.