can only modify an image if it contains a bitmap？

那么你可以通过一个方式进行变通，然后就没有问题了，如何弄呢？

看代码吧：（够长了点）
unit Unit1;


interface


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


type
  TForm1 = class(TForm)
    Image1: TImage;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    procedure MyDraw;
  public
    { Public declarations }
  end;


var
  Form1: TForm1;


implementation


{$R *.dfm}


procedure TForm1.Button1Click(Sender: TObject);
begin
  MyDraw;
end;


procedure TForm1.MyDraw;
var
  bf: BLENDFUNCTION;
  desBmp, srcBmp: TBitmap;
  rgn: HRGN;
begin
  with bf do
  begin
    BlendOp := AC_SRC_OVER;
    BlendFlags := 0;
    AlphaFormat := 0;
    SourceConstantAlpha := 180; // 透明度，0~255
  end;
 
  desBmp := TBitmap.Create;
  srcBmp := TBitmap.Create;
 
  try
    srcBmp.Assign(image1.Picture.Graphic);
 
    desBmp.Width := srcBmp.Width;
    desBmp.Height := srcBmp.Height;
 
    Windows.AlphaBlend(desBmp.Canvas.Handle, 0, 0,
      desBmp.Width, desBmp.Height, srcBmp.Canvas.Handle,
      0, 0, srcBmp.Width, srcBmp.Height, bf);
 
    rgn := CreateEllipticRgn(200,0, 500, 400); // 创建一个圆形区域
    SelectClipRgn(srcBmp.Canvas.Handle, rgn);
    srcBmp.Canvas.Draw(0, 0, desBmp);
    image1.Picture.Assign(nil);
    image1.Picture.Bitmap.Assign(nil);
    image1.Picture.Bitmap.Assign(srcBmp);
  finally
    desBmp.Free;
    srcBmp.Free;
  end
end;


procedure TForm1.FormCreate(Sender: TObject);
var
  jp:TJPEGImage;
begin
   jp:=TJPEGImage.Create;
   jp.loadfromFile('1.jpg');
   Image1.Picture.Assign(jp);
   jp.Free;
   DoubleBuffered := True;
end;


end.