Some theme, such as centum, allow you to upload custom background images to further customize your site.
Adding a background image
Go to: Add content > background image
Give it a title that makes it clear which image this is, and then click the field to choose your file. Click the upload button beside the field, and ocne the upload has completed, save the page.
Selecting a background image
Go to: Settings & admin > content, then click on the "background images" tab.
This will list your options for background images. The path with * means it will display on every page. This is the standard option, and it's not recommended to change backgrounds for other pages unless you are comfortable with url alias and display options. It's also better for site unity to keep one background for all pages.
Use the dropdown box under "background image node" to select your new background. Save your changes.
You can also click "remove" if you want to go back the the default background options within the theme.
Viewing available background images
Go to: Settings & admin > content
Then, use the filters to look for type = background image.
This will list all of the "background image" content within the site. You can edit them, or you can delete them from this page using the links on the far right.