How Do I change Rectangle Fill from code

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C#

WPF

Hi,

I have a button with a gray image in a rectangle. What I want, is that when I click on it, the button change to a blue image.

How Do I do That?

Any suggestion will be appreciated

<Button x:Name="botonAlumnos" DockPanel.Dock="Top" Click="botonAlumnos_Click">
   <Button.Template>
        <ControlTemplate>
           <Rectangle x:Name="rect1" Height="125" Width="125">
              <Rectangle.Fill>
                 <ImageBrush ImageSource="{DynamicResource _Alumnos}"/>
              </Rectangle.Fill>
           </Rectangle>
        </ControlTemplate>
   </Button.Template>
</Button>

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Now I founded this on the web

private void botonAlumnos_Click(object sender, RoutedEventArgs e)
{
 var rect1 = botonAlumnos.Template.FindName("rectAlumnos", botonAlumnos) as Rectangle;
 rect1.Fill = Brushes.White;
}

With this code the Rectangle turns White. What I want is fill it with a picture.