Dragging images

HTML

<canvas id="canvas"></canvas>

CSS

canvas {
    border: 1px solid black;
}

JavaScript

var c = document.getElementById("canvas");
var ctx = c.getContext("2d");
c.height = 400;
c.width = 400;
var images = [];
var mouseX;
var mouseY;
var highestZ = 0;
var imageOffSetX = 0;
var imageOffSetY = 0;

c.addEventListener("mousedown", selectPiece, false);
c.addEventListener("mouseup", deSelectPiece, false);
c.addEventListener("mousemove", movePiece, false);

//my image object, stores the x- and y-coordinates as well as the layering
function imgObject (x,y,z,s) {
    this.x = x;
    this.y = y;
    this.z = z;
    this.active = false; //if it's being dragged
    this.img = new Image();
    this.img.src = s;
    
    highestZ = images.length-1;
}

//creating the images
images[0] = new imgObject(100,100,0,"http://icons.iconarchive.com/icons/martin-berube/flat-animal/128/bunny-icon.png");


function movePiece(event) {
	var x = new Number();
	var y = new Number();
	var canvas = document.getElementById("canvas");
	if (event.x != undefined && event.y != undefined)
	{
	  x = event.x;
	  y = event.y;
	}
	else // Firefox method to get the position
	{
	  x = event.clientX + document.body.scrollLeft +
		  document.documentElement.scrollLeft;
	  y = event.clientY + document.body.scrollTop +
		  document.documentElement.scrollTop;
	}

	x -= canvas.offsetLeft;
	y -= canvas.offsetTop;
	mouseX = x;
	mouseY = y
	for (var i=0; i<images.length; i++) {
		if (images[i].activity === true) {
			images[i].x = x-imageOffSetX;
			images[i].y = y-imageOffSetY;
            
            if (images[i].x+images[i].img.width/2 > c.width) {
                c.width = images[i].x+images[i].img.width/2;
            }
            else if (images[i].y+images[i].img.height/2 > c.height) {
                c.height = images[i].y+images[i].img.height/2;
            }
		}
	}
}

function selectPiece(event) {
	var savedZ=0;
	var savedI=-1;
	for (var i=images.length-1; i>=0; i--) {
		images[i].activity = false;
		if (mouseX > images[i].x-0.5*images[i].img.width && 
            mouseX <...