public class Anim1 extends Applet implements Runnable {

    Thread my_thread = null;
    Image img;

    public void init() {
	img = getImage(getDocumentBase(),"foo.gif");
        my_thread = new Thread(this);
	my_thread.start();
    }

    int x=0,y=0,xdir=2,ydir=3;

    public void paint(Graphics g) {
        g.drawImage(img, x,y,null);
    }

    public void run() {
        while (true) {
	    x += xdir;
	    if (x<2 || x>size().width-20) { xdir = -xdir; }
	    y += ydir;
	    if (y<2 || y>size().height-20) { ydir = -ydir; }
	    repaint();
            try {
                my_thread.sleep(20);
            } catch (InterruptedException e) {}
        }
    }
}

Source