If System.Web.UI.WebControls.MenuRenderingMode is set to List, or if System.Web.UI.WebControls.MenuRenderingMode is set to Default and System.Web.Configuration.PagesSection.ControlRenderingCompatibilityVersion is set to 4.0 or later, this property has no effect. Use the Menu.DynamicEnableDefaultPopOutImage property or the Menu.StaticEnableDefaultPopOutImage property instead of this property.

If System.Web.UI.WebControls.MenuRenderingMode is set to Table, or if System.Web.UI.WebControls.MenuRenderingMode is set to Default and System.Web.Configuration.PagesSection.ControlRenderingCompatibilityVersion is set to 3.5, use this property to specify a custom image that is displayed in a menu item to indicate that the menu item has a dynamic submenu. This image can be in any file format (.jpg, .gif, .bmp, and so on), as long as the client's browser supports that format.

You can also disable this image in a static menu by setting this property to an empty string and setting the Menu.StaticEnableDefaultPopOutImage property to false. Similarly, you can disable this image in a dynamic menu by setting this property to an empty string and setting the Menu.DynamicEnableDefaultPopOutImage property to false.