<Window.Resources>
　　<ImageBrush x:Key="TileBrush" x:Name="DynamicBrush" TileMode="Tile"
　　　　ViewportUnits="Absolute" Viewport="0 0 32 32"
　　　　ImageSource="happyface.jpg" Opacity="0.3"></ImageBrush>
</Window.Resources>
<StackPanel Margin="5">
　　<Button Background="{DynamicResource TileBrush}" Padding="5"
　　　　FontWeight="Bold" FontSize="14" Margin="5">Uses a Dynamic Resource</Button>

　　<Button Padding="5" Margin="5" Click="cmdChange_Click"
　　　　FontWeight="Bold" FontSize="14">Change the Brush</Button>
　　<Button Background="{StaticResource TileBrush}" Padding="5" Margin="5"
　　　　FontWeight="Bold" FontSize="14">Uses a Static Resource</Button>

</StackPanel>

private void cmdChange_Click(object sender, RoutedEventArgs e)
{
    this.Resources["TileBrush"] = new SolidColorBrush(Colors.LightBlue);
}