Sorry, I may not be following. Can't you just do:
procedure TMainForm.ImageEnView1MouseWheel(Sender: TObject; Shift: TShiftState;
    WheelDelta: Integer; MousePos: TPoint; var Handled: Boolean);
begin
  if WheelDelta < 0 then
    fCurrentImage := max( 0, fCurrentImage - 1 )
  else
    fCurrentImage := min( ImageEnMView1.ImageCount - 1, fCurrentImage - 1 );
  ImageEnMView1.CopyToIEBitmap( fCurrentImage, ImageEnView1.IEBitmap );
  ImageEnView1.Update;
end;
Ensure you set ImageEnView1.MouseWheelParams.Action := iemwNone;
Nigel 
Xequte Software
www.imageen.com