var
 Bitmap: TBitmap;
 Source: TRect;
 Dest: TRect;
begin
 Bitmap := TBitmap.Create;
 try
 with Bitmap do
 begin
 Width := MyPaintBox.Width;
 Height := MyPaintBox.Height;
 Dest := Rect(0, 0, Width, Height);
 end;
 with MyPaintBox do
 Source := Rect(0, 0, Width, Height);
 Bitmap.Canvas.CopyRect(Dest, MyPaintBox.Canvas, Source);
 Bitmap.SaveToFile('MYFILE.BMP');
 finally
 Bitmap.Free;
 end;
end;