Michel,

You can use the ToolboxBitmap attribute to define the custom bitmap. The ToolboxBitmapAttribute class offers a number of constructors including using a embedded resource. The following is taken from the MSDN documentation:-

namespace NamespaceA
{
   [ToolboxBitmap(typeof(ControlA), "MyBitmap.bmp")]
   public class ControlA : System.Windows.Forms.UserControl
   {
      ...
   }
   ...
}

Regards
Neil