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if OpenPictureDialog1.Execute then
Image1.Picture.LoadFromFile(OpenPictureDialog1.FileName);					

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uses jpeg;

procedure JpegToBmp(const Filename: String);
var
 jpeg: TJPEGImage;
 bmp: TBitmap;
begin
 jpeg:=TJPEGImage.Create;
 try
   jpeg.LoadFromFile(Filename);
   bmp:=TBitmap.Create;
   try
     bmp.Assign(jpeg);
     bmp.SaveToFile(ChangeFileExt(Filename, '.bmp'));
   finally
     bmp.free;
   end;
 finally
   jpeg.free;
 end;
end;
Aufgerufen werden kann die Prozedur beispielsweise so:

procedure TForm1.Button1Click(Sender: TObject);
begin
 if OpenDialog1.Execute then
   JpegToBmp(OpenDialog1.Filename);
end;