To make the control even easier to work with, we have provided a set of built-in icons that can be 
        displayed on the button.
        Simply set the predefined CSS class to the PrimaryIconCssClass or/and SecondaryIconCssClass 
        property. All of the predefined classes are shown in the example 
        and can be used for reference.
    
    
        For example, setting PrimaryIconCssClass='rbOk' will render the embedded OK icon on the 
        left side of the button control.
    
    
        Tip: You can see how the icons are ordered in the image sprite and get their respective 
        positions here:
        Image Sprite. The icons 
        background positions are defined in the base stylesheet of Telerik RadButton control.