// Accepts drag/drop. Draws a line to signal where to insert images
procedure TMainForm.ImageEnMView1DragOver(Sender, Source: TObject;
  X, Y: Integer; State: TDragState; var Accept: Boolean);
var
  im: Integer;
  imgX, imgY: Integer;
begin
  if Source = ImageEnMView1 then
  begin
    Accept := true;
    im := ImageEnMView1.InsertingPoint(X, Y);
    imgX := ImageEnMView1.ImageX[im];
    imgY := ImageEnMView1.ImageY[im];
    ImageEnMView1.Paint;
    with ImageEnMView1.GetCanvas do
    begin
      Pen.Color := clRed;
      Pen.Style := psDot; // display a dotted line
      Pen.Width := 1;  // a dotted line only works if pen width is 1 otherwise it will be a solid line
      // draw the line vertically at the insert point
      MoveTo(imgX, imgY);
      LineTo(imgX, imgY + ImageEnMView1.ThumbHeight - 10);
    end;
  end;
end;