Modify like this if you have your original image in player.original.. Onkeyup event you should use like this ...
function onKeyUp(event)
{
switch (event.keyCode) {
case 37: case 39: player.vx = 0; player.image = player.original; break; // left or right key released
case 38: player.vy = 0; break; // up or down key released
}
}