//В UserControl

public static readonly DependencyProperty AlterImageSourceProperty= 
        DependencyProperty.Register(
        "AlterImageSource", typeof(ImageSource), typeof(ToolTipTwoStateImgButton), new PropertyMetadata(null));
			
		public static void SetAlterImageSource(UIElement element, ImageSource value)
        {
            element.SetValue(AlterImageSourceProperty, value);
        }
        public static ImageSource GetAlterImageSource(UIElement element)
        {
            return (ImageSource)element.GetValue(AlterImageSourceProperty);
        }

// В MainWindow
public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

			MessageBox.Show(Block.AlterImageSource.ToString());
        }

        
        
    }