I just saw you wanted an image, which you can do instead of color. The code is similar:
body.page-id-XX {
background: url("/wp-content/themes/yourTheme/path-to-image/image.png") !important;
}
Substitute the “XX” for the page ID for your selected page. The URL path is for the image you wish to use for the background. Make sure that is is properly formatted and optimized.