对Jpg文件的不兼容

首先看一下帮助：
TOpenPictureDialog displays a modal Windows dialog box for selecting and opening graphics files. This component is just like TOpenDialog, except that it includes a rectangular preview region. If the selected image can be read by TPicture, it is displayed in the preview region; supported file types include bitmap (.BMP), icon (.ICO), Windows metafile (.WMF), and enhanced Windows metafile (.EMF). If the selected image cannot be displayed, ?None)?appears in the preview region.If the user selects a file of an unrecognized type, TPicture raises an EInvalidGraphic exception.再看看TPicture：Use LoadFromFile to read a picture from disk. The TGraphic class created is determined by the file extension of the file. If the file extension is not recognized an EInvalidGraphic exception is raised.出错时正是EInvalidGraphic exception.所以说是因为TPicture不支持jpeg格式的图像。使用TJpegImage,就行了
unit jpegdemo;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, jpeg, ExtCtrls, FileCtrl, ComCtrls, Menus, printers;type
  TForm1 = class(TForm)
    Panel1: TPanel;
    DirectoryListBox1: TDirectoryListBox;
    FileListBox1: TFileListBox;
    Panel3: TPanel;
    DriveComboBox1: TDriveComboBox;
    Scale: TComboBox;
    PixelFormat: TComboBox;
    ColorSpace: TComboBox;
    Performance: TComboBox;    ProgressiveDisplay: TCheckBox;
    IncrementalDisplay: TCheckBox;
    MainMenu1: TMainMenu;
    File1: TMenuItem;
    Open1: TMenuItem;
    N1: TMenuItem;
    Print1: TMenuItem;
    PrinterSetup1: TMenuItem;
    N2: TMenuItem;
    Exit1: TMenuItem;
    OpenDialog1: TOpenDialog;
    PrinterSetupDialog1: TPrinterSetupDialog;
    PrintDialog1: TPrintDialog;
    ScrollBox1: TScrollBox;
    Image1: TImage;
    procedure FileListBox1DblClick(Sender: TObject);    procedure SetJPEGOptions(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure ProgressUpdate(Sender: TObject; Stage: TProgressStage;
      PercentDone: Byte; RedrawNow: Boolean; const R: TRect; const Msg: string);
    procedure Open1Click(Sender: TObject);
    procedure Print1Click(Sender: TObject);
    procedure PrinterSetup1Click(Sender: TObject);
    procedure Exit1Click(Sender: TObject);  private
    { Private declarations }
  public
    { Public declarations }
    procedure OpenFile(const Filename: string);
  end;var
  Form1: TForm1;implementation{$R *.DFM}procedure TForm1.OpenFile(const Filename: string);
begin
  try
    Image1.Picture.LoadFromFile(Filename);
  except
    on EInvalidGraphic do
      Image1.Picture.Graphic := nil;  end;
  SetJPEGOptions(self);
end;procedure TForm1.FileListBox1DblClick(Sender: TObject);
begin
  OpenFile(FileListbox1.Filename);
end;procedure TForm1.SetJPEGOptions(Sender: TObject);
var
  Temp: Boolean;
begin
  Temp := Image1.Picture.Graphic is TJPEGImage;
  if Temp then
    with TJPEGImage(Image1.Picture.Graphic) do
    begin
      PixelFormat := TJPEGPixelFormat(Self.PixelFormat.ItemIndex);      Scale := TJPEGScale(Self.Scale.ItemIndex);
      Grayscale := Boolean(Colorspace.ItemIndex);
      Performance := TJPEGPerformance(Self.Performance.ItemIndex);
      ProgressiveDisplay := Self.ProgressiveDisplay.Checked;
    end;
  Scale.Enabled := Temp;
  PixelFormat.Enabled := Temp;
  Colorspace.Enabled := Temp;
  Performance.Enabled := Temp;
  ProgressiveDisplay.Enabled := Temp
    and TJPEGImage(Image1.Picture.Graphic).ProgressiveEncoding;  Image1.IncrementalDisplay := IncrementalDisplay.Checked;
end;procedure TForm1.FormCreate(Sender: TObject);
begin
  Scale.ItemIndex := 0;
  PixelFormat.ItemIndex := 0;
  Colorspace.ItemIndex := 0;
  Performance.ItemIndex := 0;
  OpenDialog1.Filter := GraphicFilter(TGraphic);
  FileListbox1.Mask := GraphicFileMask(TGraphic);
  Image1.OnProgress := ProgressUpdate;
end;procedure TForm1.ProgressUpdate(Sender: TObject; Stage: TProgressStage;  PercentDone: Byte; RedrawNow: Boolean; const R: TRect; 
const Msg: string);
begin
  if Stage = psRunning then
    Caption := Format('%d%%',[PercentDone])
  else
    Caption := 'Form1';
end;
procedure TForm1.Open1Click(Sender: TObject);
begin
  if OpenDialog1.Execute then
    OpenFile(OpenDialog1.FileName);
end;procedure TForm1.Print1Click(Sender: TObject);var
  AspectRatio: Single;
  OutputWidth, OutputHeight: Single;
begin
  if not PrintDialog1.Execute then Exit;
  Printer.BeginDoc;
  try
    OutputWidth := Image1.Picture.Width;
    OutputHeight := Image1.Picture.Height;
    AspectRatio := OutputWidth / OutputHeight;
    if (OutputWidth < Printer.PageWidth) and
      (OutputHeight < Printer.PageHeight) then
    begin
      if OutputWidth < OutputHeight then      begin
        OutputHeight := Printer.PageHeight;
        OutputWidth := OutputHeight * AspectRatio;
      end
      else
      begin
        OutputWidth := Printer.PageWidth;
        OutputHeight := OutputWidth / AspectRatio;
      end
    end;
    if OutputWidth > Printer.PageWidth then
    begin
      OutputWidth := Printer.PageWidth;
      OutputHeight := OutputWidth / AspectRatio;
    end;
    if OutputHeight > Printer.PageHeight then    begin
      OutputHeight := Printer.PageHeight;
      OutputWidth := OutputHeight * AspectRatio;
    end;
    Printer.Canvas.StretchDraw(Rect(0,0,
      Trunc(OutputWidth), Trunc(OutputHeight)),
      Image1.Picture.Graphic);
  finally
    Printer.EndDoc;
  end;
end;procedure TForm1.PrinterSetup1Click(Sender: TObject);
begin
  PrinterSetupDialog1.Execute;
end;procedure TForm1.Exit1Click(Sender: TObject);begin
  Close;
end;end.