...Read(HeaderStr)
...Copy(HeaderStr, 1, 2) = 'BM' ==> BMP
...Copy(HeaderStr, 2, 3) = 'PNG'===>PNG
...Copy(HeaderStr, 1, 3) = 'GIF' ==>GIF
...Copy(HeaderStr, 1, 4) = 'JFIF'==>JPG

不保证对。哈。