XAML:
        <Image Source="{Binding ElementName=DockItemControl, Path=TestProp}" x:Name="img"/>

Code-Behind:
        public String TestProp
        {
            get { return (String)GetValue(TestPropProperty); }
            set { SetValue(TestPropProperty, value); }
        }

        public static readonly DependencyProperty TestPropProperty =
            DependencyProperty.Register("TestProp", typeof(String), typeof(DockItem), new UIPropertyMetadata(0));



Window-Code:
        <local:DockItem HorizontalAlignment="Left" TestProp="Test"/>