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var frames: array [0..10000] of tbitmap;					

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procedure TfrmCounter.Timer2Timer(Sender: TObject);
var i: integer;
begin
            begin
            frames[counter] := tbitmap.create;

            frmIMG.ListBox1.Items.Add(IntToStr(counter));
            //in der Form frmIMG möchte ich dann auf die Ausschnitte zugreifen
   
                  try
                  ScreenShot(DesktopBitmap);
                  finally
                  DesktopBitmap.FreeImage;

                  lBitmap.canvas.draw(-min(X1,y1),-min(x2,y2),desktopbitmap);
                  frames[counter].assign (lBitmap);
                  inc (counter);
                  end;
            if counter = 10 then
            begin
            timer2.enabled := false;
            counter := 0;
            frmIMG.Show;
          
            end;
end;
end;					

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procedure TfrmIMG.ListBox1Click(Sender: TObject);
var
i: Integer;
begin
  for i:=0 to high(frames)-1 do image1.picture.bitmap.assign (frames[i]);
end;					

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frmIMG.image1.picture.bitmap.assign(frames[5]);					

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Image1.Picture.Bitmap.Assign(frames[ListBox1.itemindex]);					

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if ListBox1.ItemIndex >= 0 then
  ...