silverlight按钮换背景图

请问silverlight下button换背景图
 <Button Content="B" Height="26" HorizontalAlignment="Left"  Name="button3" VerticalAlignment="Stretch" Width="75" Margin="10,165,0,137" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" FlowDirection="LeftToRight">  <Button Content="B