Only some layouts support background image customization and properties.
If you have a Paid, Permanent, or Early Adopter account, you can create a custom layer to add a background image or properties to a layout that does not support it via the S2 customization area.
Actually there's a middle option now. You do not have to have a paid account in order to change the properties of the background images if the layout doesn't support them. All the users can use the Custom CSS section to enter the appropriate CSS, including the properties. This option is available on all the system layouts.
Aaand, I suppose I should edit this s2howto tutorial, so it can be linked from this FAQ.