Dynamic Image (cache?) problem/bug

<?xml version="1.0" encoding="UTF-8"?>
<zk xmlns="http://www.zkoss.org/2005/zul">
    <?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" root="./win"?>
    <window id="win" title="Dynamic Image" border="normal" width="100%" height="100%"
    apply="controller.IndexController">
		
        <vbox hflex="1">
            <label value="Double-click on an icon to choose it."  style="font-size: 18px"/>
            <label value="You will be given new icons to choose from."  style="font-size: 18px"/>
            <label value="See if the new icons have the numbers they should have,"  style="font-size: 18px"/>
            <label value="and if the tooltip matches the number on the icon."  style="font-size: 18px" />

            <separator bar="true" height="15px" />

            <hbox>
                <label value="Choices (should be):" style="font-size: 18px" />
                <label id="correctChoices" style="font-size: 24px; color: red" />
            </hbox>
            <groupbox id="choicesArea" mold="3d" hflex="1" contentStyle="overflow:auto" ></groupbox>

            <separator bar="true" height="15px" />
            <label value="Picked:" style="font-size: 18px" />
            <groupbox id="pickedArea" mold="3d" hflex="1" droppable="true" height="200px"></groupbox>
        </vbox>
    </window>
</zk>

package controller;

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.List;
import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.event.DropEvent;
import org.zkoss.zk.ui.event.Event;
import org.zkoss.zk.ui.event.EventListener;
import org.zkoss.zk.ui.event.Events;
import org.zkoss.zk.ui.util.GenericForwardComposer;
import org.zkoss.zul.Groupbox;
import org.zkoss.zul.Image;
import org.zkoss.zul.Label;
import org.zkoss.zul.Space;

public class IndexController extends GenericForwardComposer {

    boolean odd = false;
    boolean odd1 = false;
    private int cnt = 1;
    private Groupbox choicesArea, pickedArea;
    private Label correctChoices;

    @Override
    public void doAfterCompose(Component comp) throws Exception {
        super.doAfterCompose(comp);

        updateNextArea();
    }

    BufferedImage newimg(boolean update, int num) {
        BufferedImage bi = new BufferedImage(150, 150, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = bi.createGraphics();
        g2d.setStroke(new BasicStroke(5));
        Line2D line = null;
        Rectangle2D retangle = null;
        if (update) {
            line = new Line2D.Double(10, 10, 130, 130);
            retangle = new Rectangle2D.Double(25, 25, 85, 85);
        } else {
            line = new Line2D.Double(10, 130, 130, 10);
            retangle = new Rectangle2D.Double(25, 25, 85, 85);
        }
        g2d.setColor(update ? Color.cyan : Color.RED);
        g2d.draw(line);
        g2d.setColor(update ? Color.yellow : Color.pink);
        g2d.draw(retangle);
        g2d.setFont(new Font(Font.SANS_SERIF, Font.PLAIN, 18));
        g2d.drawString(Integer.toString(num), 55, 50);

        return bi;
    }

    public void updateNextArea() {

        Image anImage;
        Space aSpace;
        List<Component> choicesAreaChildren;
        List<Component> choicesAreaKids;
        int numChoices = 4;
        String correctChoicesValue = "";

        choicesAreaChildren = choicesArea.getChildren();
        choicesAreaKids = new ArrayList<Component>();

        // copy the list to prevent ConcurrentModificationException
        for (Component comp : choicesAreaChildren) {
            choicesAreaKids.add(comp);
        }

        // clear current choices
        for (Component comp : choicesAreaKids) {
            if ((comp.getClass() == Image.class) || (comp.getClass() == Space.class)) {
                comp.detach();
            }
        }

        // generate a new set of icons with the next numChoices numbers
        for (int i = 0; i < numChoices; i++) {
            anImage = new Image();
            anImage.setContent(newimg((odd1 = !odd1), cnt));

            anImage.setTooltiptext("image number: " + cnt);
            correctChoicesValue += cnt + " ";
            cnt++;

            anImage.setDraggable("true");
            anImage.addEventListener(Events.ON_DOUBLE_CLICK, new EventListener() {

                @Override
                public void onEvent(Event event) throws Exception {
                    DropEvent dropEvent = new DropEvent(Events.ON_DROP, pickedArea, event.getTarget(), 0, 0, 0, 0, 0);
                    Events.sendEvent(dropEvent);
                }
            });
            aSpace = new Space();
            aSpace.setSpacing("10px");
            choicesArea.appendChild(anImage);
            choicesArea.appendChild(aSpace);
        }
        correctChoices.setValue(correctChoicesValue);
    }

    public void onDrop$pickedArea(DropEvent evt) {
        Space aSpace;
        Image toolImg = (Image) evt.getDragged();

        move(evt.getTarget(), evt.getDragged());
        toolImg.setDraggable("false");
        aSpace = new Space();
        aSpace.setSpacing("10px");
        evt.getTarget().appendChild(aSpace);

        updateNextArea();
    }

    private void move(Component dropTo, Component dragged) {
        dropTo.appendChild(dragged);
    }
}