Menu Background and Background Image
Please use the Visual Style Builder to edit Background gradient colors of the the RadDropDownMenuElement >> FillPrimitive.
Menu Background Image
The RadMenu's BackgroundImage and BackgroundImageLayout properties let you specify an image to be tiled, zoomed, centered, or stretched across the portion of the menu not covered by menu items.
To provide an image to the background of RadMenu, use a standard Windows Panel control, set the BackGroundImage property to the image for your menu and finally set the RadMenu.BackColor property to Transparent. See the Demo application >> Menu >> Menu First look example that ships with the Telerik UI for WinForms.