We have made changing the background extremely easy with the front end live designer.
When you're in live design's Design Mode, the starting mode is Basic Design. This mode gives handy quick styles that are most commonly changed styles.
To change the background of your page click on Background Color or Background Image in the Body section.
If you want a solid color, click Background Color, in the dropdown select "define (hex)" and then you can either enter the color value or use the color picker to select the color.
If you want to use a background image, click Background Image, and select url. If you know the path to the image you can simply type the path in the textbox.
If you do not know the path you can click the "Browse" link to open the file browser to browse the server for a file (you will be defaulted to the root of the site).
Simply navigate to the folder, select your image and click insert.
If you do not have your image uploaded, click the Upload Files tab at the top, browse for the file on your computer, select a destination folder and upload. Once uploaded, you can go back to the Server Files tab and select it from the folder you uploaded it to.