uses pngimage;  

{从 png 图片到十六进制字符串}

function Png2Hex(png: TPngImage): string;

var

  stream: TMemoryStream;

begin

  stream := TMemoryStream.Create;

  png.SaveToStream(stream);

  SetLength(Result, stream.Size * 2);

  BinToHex(stream.Memory, PChar(Result), stream.Size);

  stream.Free;

end;  

{从十六进制字符串恢复 png 图片}

procedure Hex2Png(str: string; out png: TPngImage);

var

  stream: TMemoryStream;

begin

  if not Assigned(png) then png := TPngImage.Create;

  stream := TMemoryStream.Create;

  stream.SetSize(Length(str) div 2);

  HexToBin(PChar(str), stream.Memory, stream.Size);

  png.LoadFromStream(stream);

  stream.Free;

end;  

{测试}

procedure TForm1.Button1Click(Sender: TObject);

var

  png: TPngImage;

begin

  png := TPngImage.Create;

  png.LoadFromFile('c:/temp/test.png');

  Memo1.Text := Png2Hex(png);

  png.Free;

end;  

procedure TForm1.Button2Click(Sender: TObject);

var

  png: TPngImage;

begin

  png := TPngImage.Create;

  Hex2Png(Memo1.Text, png);

  Canvas.Draw(0, 0, png);

  png.Free;

end;