procedure TForm1.Button1Click(Sender: TObject);
var xbmp : TBitmap ;
begin
   xbmp:=TBitmap.Create;
   with xbmp do begin
      pixelformat:=pf24bit;
      Canvas.Brush.color:=clBlack;
      Canvas.Brush.Style:=bsSolid;
      width:=image1.Width;
      height:=image1.height;
      Canvas.Brush.Color:=clWhite;
      Canvas.Ellipse(Canvas.ClipRect);
   end;
   form1.Canvas.CopyRect(image1.Picture.Bitmap.Canvas.ClipRect,
                         image1.Picture.Bitmap.Canvas,
                         image1.Picture.Bitmap.Canvas.ClipRect);
   form1.Canvas.CopyMode:=cmSrcAnd;
   form1.Canvas.CopyRect(xbmp.Canvas.ClipRect,
                          xbmp.Canvas,
                          xbmp.Canvas.ClipRect);
   xbmp.Free;
end;