DPI其实和图片本身没有多大关系，和显示出来的效果有关系

下面设置JPEG 的DPI参数

procedure SetJpgdpi(filename: string; dpix, dpiy: Integer);

const

  BufferSize = 50;

  DPI = 1; {inch}

  DPC = 2; {cm}

var

  Buffer: string;

  index: Integer;

  FileStream: TFileStream;

  xResolution: WORD;

  yResolution: WORD;

  typeA: Byte;

begin

  FileStream := TFileStream.Create(filename, fmOpenReadWrite);

  try

     SetLength(Buffer, BufferSize);

     FileStream.Read(buffer[1], BufferSize);

     index := Pos('JFIF' + #$00, buffer);

     if index > 0 then

        begin

           FileStream.Seek(index + 6, soFromBeginning);

           typeA := DPI;

           FileStream.write(typeA, 1);

           xresolution := swap(dpix);

           FileStream.write(xresolution, 2);

           yresolution := swap(dpiy);

           FileStream.write(yresolution, 2);

        end

  finally

     FileStream.Free;

  end;

end;