<Border BorderBrush="Black" BorderThickness="1.5" CornerRadius="12">
    <Border.Background>
        <ImageBrush>
            <ImageBrush.ImageSource>
                <BitmapImage UriSource="c:\temp\happy.jpg" />
            </ImageBrush.ImageSource>
        </ImageBrush>
    </Border.Background>
</Border>

//używając metod anonimowych
var worker1 = new System.ComponentModel.BackgroundWorker();
worker1.DoWork += (sender,e) => Thread.Sleep(10000);
worker1.RunWorkerCompleted += (sender,e) => MessageBox.Show("Worker1 Finished!");
worker1.RunWorkerAsync();