It's not entirely clear what you're trying to do. When you refer to checkbox do you mean the iconbutton is your checkbox?
You've defined both images so you should see it changing when you click the button, but for it to work as a checkbox you need to enable its toggle mode:
{style:'toolbutton', toggle:true}
Otherwise, if you wanted to change the icon based on a separate checkbox control you could just change it in that checkbox's onClick function using:
button.image = someOtherImage;
but that only changes the "off" icon, I'm not sure how you would access the other one.