判断头几个字节：

function IsJpegFile(FileName: string): Boolean;
const
RightBuf : array[0..3] of Byte = ($FF,$D8,$FF,$D9);
var
Buf: array[0..3] of Byte;
begin
FillChar(Buf, 4, 0);
with TFileStream.Create(FileName, 0) do begin
Position := 0;
ReadBuffer(Buf[0], 2);
Position := Size-2;
ReadBuffer(Buf[2], 2);
Free;
end;
Result := CompareMem(@RightBuf[0], @Buf[0], 4);
end;

procedure TForm1.Button1Click(Sender: TObject);//测试
begin
if Self.OpenDialog1.Execute then
if IsJpegFile(Self.OpenDialog1.FileName) then
Showmessage(‘Is Jpg File‘);
end;

http://blog.csdn.net/diligentcatrich/article/details/6367077

Delphi判断一个文件是不是JPG图片