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uses
  [...], JPEG;

procedure ResizeImage(FileName: String; NewWidth, NewHeight: Integer);
var
  RBit, Bit: TBitmap;
  JPG: TJPEGImage;
begin
  Bit:=TBitmap.Create;
  RBit:=TBitmap.Create;
  JPG:=TJPEGImage.Create;
  JPG.LoadFromFile(FileName);
  Bit.Assign(JPG);
  RBit.Width:=NewWidth;
  RBit.Height:=NewHeight;
  RBit.Canvas.CopyRect(Rect(0,0,NewWidth,NewHeight),Bit.Canvas,Rect(0,0,Bit.Width,Bit.Height);
  JPG.Assign(RBit);
  JPG.SaveToFile(FileName);
  JPG.Free;
  RBit.Free;
  Bit.Free;
end;