For the time being the following JavaScript workaround can be used:
JavaScript:
		function pageLoad() {
			fixImageURL();
		}
		function fixImageURL() {
			var imageBtns = $telerik.$('.rbImageButton');
			for (var i = 0; i < imageBtns.length; i++) {
				var btnStyle = imageBtns[i].getAttribute('style').replace("background-image:url(", "background-image:url('").replace(");", "');");
				imageBtns[i].setAttribute('style', btnStyle);
			}
		}
ASPX:
		<telerik:RadButton ID="RadButton1" runat="server" Width="50px" Height="50px">
			<Image ImageUrl="50x50.png" />
		</telerik:RadButton>
		<telerik:RadButton ID="RadButton2" runat="server" Width="50px" Height="50px">
			<Image ImageUrl="50x50.png" />
		</telerik:RadButton>