... You don't have to specify an area but then the whole area will get repainted. That is not necessarily a problem but can lead to flickering, as the repainting process is relatively slow.
Since Pictureboxes are always double buffered by default, you shouldn't get flickering even when doing a full repaint, as long as you do repaint everything.
"Anyone can do any amount of work, provided it isn't the work he is supposed to be doing at that moment" Robert Benchley, 1930