I am not sure if I got you right but I think you have to possibilites:
Either set the Icon of the ParentForm.
Or set the ApplicationIcon as described here: http://msdn.microsoft.com/en-us/library/339stzf7.aspx
EDIT:
As you are using a control as TopLevelControl you need to send the WM_SETICON during the creation of the control - as the form does!
Taken from the Form.CreateHandle:
Icon icon = this.Icon;
if (icon != null && this.TaskbarOwner.Handle != IntPtr.Zero)
{
UnsafeNativeMethods.SendMessage(this.TaskbarOwner, 128, 1, icon.Handle);
}