function TForm2.GetImageCountPerRow(const AImageEnMView: TImageEnMView): Integer;
var
  i: Integer;
begin
  Result := -1;

  for i := 0 to AImageEnMView.ImageCount - 1 do
  begin
    if AImageEnMView.ImageRow[i] = 1 then
    begin
      Result := i;
      BREAK;
    end;
  end;
end;

function TForm2.GetIndexOfFirstVisibleImage(const AImageEnMView: TImageEnMView): Integer;
var
  c: Integer;
begin
  c := GetImageCountPerRow(AImageEnMView);

  if c = -1 then
  begin
    // not enough images to fill a row + 1
    Result := 0;
  end
  else
    Result := (AImageEnMView.ViewY div AImageEnMView.ThumbHeight) * c;
end;

CodeSite.Send('Index of first visible Image', GetIndexOfFirstVisibleImage(ImageEnMView1));

function TForm2.GetImageCountPerRow(const AImageEnMView: TImageEnMView): Integer;
var
  i: Integer;
begin
  Result := -1;

  for i := 0 to AImageEnMView.ImageCount - 1 do
  begin
    if AImageEnMView.ImageRow[i] = 1 then
    begin
      Result := i;
      BREAK;
    end;
  end;
end;

function TForm2.GetIndexOfFirstVisibleImage(const AImageEnMView: TImageEnMView): Integer;
var
  c: Integer;
begin
  c := GetImageCountPerRow(AImageEnMView);

  if c = -1 then
    Result := -1
  else
    Result := (AImageEnMView.ViewY div AImageEnMView.ThumbHeight) * c;
end;

CodeSite.Send('Index of first visible Image', GetIndexOfFirstVisibleImage(ImageEnMView1));

function GetImageCountPerRow: Integer;
var
  i: Integer;
begin
  Result := -1;

  for i := 0 to ImageEnMView1.ImageCount - 1 do
  begin
    if ImageEnMView1.ImageRow[i] = 1 then
    begin
      Result := i;
      BREAK;
    end;
  end;
end;

CodeSite.Send('Index of first visible Image', 
  (ImageEnMView1.ViewY div ImageEnMView1.ThumbHeight) * GetImageCountPerRow);