The property DisabledImageUrl
of the RadButton will be included in the upcoming Q3 2011 Beta release of the RadControls for ASP.NET AJAX, scheduled for Oct 25, 2011.
For the time being, you can set the background image of the button control in disabled state through its DisabledButtonCssClass
I hope the provided information helps. Don't hesitate to contact us again if you encounter additional difficulties.
the Telerik team
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