In order to achieve the desired behavior you should set the desired background to the TD element which represents the RadSlidingZone.
The easiest way to do it is by using CSS means as shown below:
| <style type="text/css">
| background: url('paneTabContainer.gif') repeat-y !important;
Please, note that you should use the RadSlidingZone's ClientID in order to apply the style to the particular zone. Also, make sure that you have add the keyword !important
because otherwise your custom style will get overridden by the embedded one.
the Telerik team