How would i get these images to change?

[pineappelle]

The code below changes the image to the next one but wont continue changing the image for all for of them. What am i doing wrong?

/**
 * @(#)chapter16random.java
 *
 * chapter16random Applet application
 *
 * @author
 * @version 1.00 2012/2/8
 */

import java.awt.*;

public class chapter17a extends java.applet.Applet implements Runnable {

	Graphics offscreen;
	Thread runner;
	int left, top, xmove;
	Image workspace;
	Image[] ball;
	int currentpicture;
	MediaTracker tracker;

	public void init() {

		workspace=createImage(400,400);
		ball=new Image[4];
		offscreen=workspace.getGraphics();
		left=10;
		top=10;
		xmove=1;
		loadimages();
		currentpicture=0;

	}

	public void loadimages() {

		tracker=new MediaTracker(this);
		ball[0]=getImage(getCodeBase(), "ball1.gif");
		ball[1]=getImage(getCodeBase(), "ball2.gif");
		ball[2]=getImage(getCodeBase(), "ball3.gif");
		ball[3]=getImage(getCodeBase(), "ball4.gif");
		tracker.addImage(ball[0],0);
		tracker.addImage(ball[1],1);
		tracker.addImage(ball[2],2);
		tracker.addImage(ball[3],3);
		try{tracker.waitForAll();
		}
		catch(InterruptedException e) {}

	}

	public void paint(Graphics screen) {

		offscreen.setColor(Color.white);
		offscreen.fillRect(0,0,400,400);
		offscreen.setColor(Color.red);

		if(currentpicture==0) {
			offscreen.drawImage(ball[0],left,top,75,75,this);
		}
		else if(currentpicture==1){
			offscreen.drawImage(ball[1],left,top,75,75,this);
		}
		else if(currentpicture==2) {
			offscreen.drawImage(ball[2],left,top,75,75,this);
		}
		else {
			offscreen.drawImage(ball[3],left,top,75,75,this);
		}

		screen.drawImage(workspace,0,0,400,400,this);

	}

	public void start() {

		if(runner==null) {
			runner=new Thread(this);
		}
		runner.start();

	}

	public void run() {

		while(true) {
			repaint();
			checkedge();
			left=left+xmove;
			try {Thread.sleep(5); }
			catch (InterruptedException e) {}
		}

	}

	public void stop() {

		if(runner!=null) {
			runner.stop();
			runner=null;
		}

	}

	public void checkedge() {

		if(left<0||left+75>400) {
			currentpicture=1;
			runner.suspend();
			try {
				Thread.sleep(10);
			}
			catch (InterruptedException e){}
			runner.resume();
			currentpicture=2;
			currentpicture=3;
			currentpicture=4;
		}

	}

	public void update(Graphics screen) {
		paint(screen);
	}
}