I know this thread is rather old, but I was looking for a solution to this issue and found one that suited my application, so I'm posting it here. It appears the original poster was attempting to set the
background image of the entire RadPanelBar control, which is also what I wanted to accomplish also. Shinu, the link you provided discusses how to set images within
the RadPanelBar's Items, which is entirely different.
My app is using a RadSplitter control with two RadPanes: a pane on the left side, which contains a RadPanelBar, and a pane on the right side, whose contents reflects the user's menu choice. I'm using a fixed height for the RadSplitter for a variety of reasons not relevant to this issue. The result is that the menu items do not completely fill the RadPane, and I'm left with a bland white rectangle underneath the menu items, filling what's left of the RadPane. (This, by the way, is the color setting of the RadPanelBar's background color for the skin I've chosen.) That bland white rectangle looks...well...bland. I couldn't find a way to jazz up the RadPanelBar's background directly, but I found that putting it inside an asp:Panel did the job. Here's the code snippet for the whole RadPane. Hopefully this can help someone else.