The sketch window in Processing is like a painting canvas. Everything is drawn on top of everything before that. So if you don't want an aggregate drawing, you should add an explicit background call to the beginning of your draw() loop that clears the screen.

With regard to updatePixels, this is used in another context. When you want to change specific pixels of the screen or an image/pgraphics, you usually call loadPixels, then change some or all pixels in the pixel array, then call updatePixels to activate these changes. Without an updatePixels you wouldn't see the changes, so in that case you need to tell Processing to update the pixels once they have been 'changed'. The reason you get a NPE here is because you haven't called loadPixels, so the main pixel array isn't available.

Extra tip: don't load your image in the draw() loop.

1.   // make this a global variable (outside setup and draw)
   
2.   PImage player;
3.   // load this in setup (= load the image once)
   
4.   player = loadImage("pwalk2.png");
5.   // display the image in draw (= as many times as you want)
   
6.   image(player, playerx, playery);