Juergen,

 

Yes, this is an example:

 

   System.Web.UI.WebControls.Image Image1 = new System.Web.UI.WebControls.Image ();
   Image1.ToolTip    = "Insert today's date";
   Image1.ImageUrl    = "tools.gif";
   Image1.CssClass    = "CuteEditorButton";
   SetMouseEvents(Image1);
   Image1.Attributes["onclick"]="var d = new Date(); CuteEditor_GetEditor(this).ExecCommand('PasteHTML',false,d.toLocaleDateString())";
   tc.Control.Controls.Add(Image1);

 

You can find the above code in the Add custom buttons example.