procedure TForm1.Button1Click(Sender: TObject);
var Bitmap: TBitmap;
begin
Bitmap := TBitmap.Create;
Bitmap.Width := PaintBox1.Width;
Bitmap.Height := PaintBox1.Height;
Bitmap.Canvas.CopyRect(PaintBox1.ClientRect, PaintBox1.Canvas, PaintBox1.ClientRect);
Bitmap.SaveToFile('c:\test.bmp');
Bitmap.Destroy;
end;