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var
  BufferImage: TImage {ist schon erstellt }

[..]
begin
  BufferImage.Picture.LoadFromFile('C:\router.bmp');
  BufferImage.Picture.Bitmap.Canvas.Pixels[Pixel.X,Pixel.Y] := clBlack; { <- Bild verschwindet vom Form }
  BufferImage.Picture.SaveToFile('C:\router.bmp');
end;

end.                    

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procedure TForm132.btn1Click(Sender: TObject);
var
  Picture: TPicture;
begin
  Picture := TPicture.Create;
  try
    Picture.LoadFromFile('C:\test.bmp');
    Picture.Bitmap.Canvas.Pixels[1, 1] := clBlack;
    Picture.SaveToFile('C:\test2.bmp');
  finally
    Picture.Free;
  end;
end;                    

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var
  Picture: TBitmap;
  JpegImg: TJPEGImage;
begin
  JpegImg := TJPEGImage.Create;
  try
    JpegImg.LoadFromFile('C:\test.jpg');
    try
      Picture := TBitmap.Create;
      Picture.Assign(JpegImg);
      Picture.Canvas.Pixels[1, 1] := clBlack;
      JpegImg.Assign(Picture);
    finally
      Picture.Free;
    end;
    JpegImg.SaveToFile('C:\test2.jpg');
  finally
    JpegImg.Free;
  end;