RadControls for ASP.NET AJAX
RadNotification provides two icons in its UI to help convey the message - the TitleIcon and ContentIcon. The TitleIcon's size is 16 by 16 pixels and the ContentIcon's size is 32 by 32 pixels. You can point these properties to a custom image in your application or you can use the embedded icons. The list with the possible values for the embedded icons is as follows:
The built-in icons work only when the embedded skins are used. If you externalize the skins or use a custom skin you cannot set them via the above shorthand values.
If you wish to use them in this case please examine this KB article on the matter.