Image leaves trail when used with canvas. (Javascript)

I am trying to make a simple HTML5 game. And the WASD keys move the character around, when he moves a trail of images follows him. I know I need to clear the canvas but I don't know where to put it, in the places I tried it the thing either didn't move or it was changing the variable but wasn't showing the character move from one place to another. A copy of my code is on JSBin