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if OpenDialog.FileName <> '*.bmp' then 
  begin
    Image.Picture.LoadFromFile(OpenDialog.FileName),
    Image.Picture.SaveToFile('temp.bmp');
    Image.Picture.LoadFromFile('temp.bmp');
  end;                    

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var jpg : TJpegImage;
    bmp : TBitmap;

// hier irgendwie jpg laden

bmp := TBitmap.Create;
bmp.Assign(jpg);
jpg.Free;

// jetzt ists umgewandelt