Draw Lines with Arrows.

 public partial class RadForm1 : Telerik.WinControls.UI.RadForm
 {
     decimal lastUsedThickness;
     public RadForm1()
     {
         InitializeComponent();
         RadMenuItem arrowButton = new RadMenuItem("DrawArrow");
         arrowButton.Click += this.ArrowButton_Click;
        radImageEditor1.ImageEditorElement.CommandsElement.CommandsStackElement.Children.Insert(14, arrowButton);
     }

     private void ArrowButton_Click(object sender, EventArgs e)
     {
         MyArrowDialog arrowDialog = new MyArrowDialog(this.radImageEditor1.ImageEditorElement);
         arrowDialog.TopMost = true;
         
         ((RadSpinEditor)arrowDialog.Controls[0].Controls["radSpinEditorBorderThickness"]).ValueChanged += spinEditor_ValueChanged;
         if (lastUsedThickness > 0)
         {
          ((RadSpinEditor)arrowDialog.Controls[0].Controls["radSpinEditorBorderThickness"]).Value = lastUsedThickness;
         }

         arrowDialog.Show();
     }

     private void spinEditor_ValueChanged(object sender, EventArgs e)
     {
         lastUsedThickness = (sender as RadSpinEditor).Value;
     }
 }

 public class MyArrowDialog : DrawShapeDialog
 {
     public MyArrowDialog(RadImageEditorElement imageEditorElement) : base(imageEditorElement)
     {  
        ((RadDropDownList) this.Controls[0].Controls["radDropDownListShape"]).SelectedIndex = 6;

         var spinEditor = this.Controls[0].Controls["radSpinEditorBorderThickness"] as RadSpinEditor;
         ((RadSpinEditor)this.Controls[0].Controls["radSpinEditorBorderThickness"]).ValueChanged += this.MyDialog_ValueChanged;
       
         this.ApplySettings();
     }

     private void MyDialog_ValueChanged(object sender, EventArgs e)
     {
         this.ApplySettings();
     }
 }