background: url("URL OF IMAGE") no-repeat scroll center top #292929;
You would replace URL OF IMAGE with the URL of the background image you uploaded to your media library. If it is a repeating image, change ‘no-repeat” to “repeat.” The hex color code at the end of the background declaration should be changed to a color that is complimentary to the image used so that just in case the image doesn’t load, people have something with a similar color value instead of just a stark white or something else.
A comment: For your background image, optimize it heavily for the web and try and keep it under 250k in file size so your site loads quickly. This is especially important with photography sites since the focus is on images, and you don’t want to keep people waiting for the background image to load.