Gets the background image of a component.
This can be an absolute or relative URL or class.
You can either have both a backgroundColor and a backgroundImage set at the same time. The background image is displayed on top of the background color. The default value is undefined, meaning "not set." If this style and the backgroundColor style are undefined, the component has a transparent background.
The default skins of most Flex controls are partially transparent. As a result, the background image of a container partially shows through to controls that are in that container. You can avoid this by setting the alpha values of the control's fillAlphas property to 1.