本文主要讲解了如何判断一个图片文件的真实类型，并不是从后缀名来判断类型的方法：

以下是引用片段：

unit Unit55;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TImageType = (IT_None, IT_Error, IT_Bmp, IT_JPEG, IT_GIF, IT_PCX, IT_PNG,
    IT_PSD, IT_RAS, IT_SGI, IT_TIFF);

  TForm55 = class(TForm)
    OpenDialog1: TOpenDialog;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form55: TForm55;

implementation

{$R *.dfm}

function CheckImageType(FileName: string): TImageType;
var
  MyImage: TMemoryStream;
  Buffer: Word;
begin
  MyImage := TMemoryStream.Create;
  try
    MyImage.LoadFromFile(FileName);
    MyImage.Position := 0;
    if MyImage.Size = 0 then // 如果文件大小等于0，那么错误(
    begin
      Result := IT_Error;
      Exit;
    end;
    MyImage.ReadBuffer(Buffer, 2); //读取文件的前２个字节,放到Buffer里面

    case Buffer of
      $4D42:
        Result := IT_Bmp;
      $D8FF:
        Result := IT_JPEG;
      $4947:
        Result := IT_GIF;
      $050A:
        Result := IT_PCX;
      $5089:
        Result := IT_PNG;
      $4238:
        Result := IT_PSD;
      $A659:
        Result := IT_RAS;
      $DA01:
        Result := IT_SGI;
      $4949:
        Result := IT_TIFF;
    else
      Result := IT_None;
    end;
  finally
    MyImage.Free;
  end;
end;

procedure TForm55.Button1Click(Sender: TObject);
begin
  if not OpenDialog1.Execute then
    Exit;
  if CheckImageType(OpenDialog1.FileName)<>IT_Bmp then
    showmessage('不是BMP格式');

end;

end.