CSS was not meant to be used like this but here is a work around.. something i call a BODGE. This will be using the :checked pseudo class - attach it to a pseudo element of a checkbox then change its background accordingly.
I have even put robot pictures in there for you ;)
CSS:
input[type="checkbox"] {
    content: url('http://www.thelemming.com/lemming/POP-CULTURE/asimo-robot_48.jpg');
    display: block;
    width: 200px;
    height: 200px;
    border: 1px solid black;
}
input[type="checkbox"]:checked {
    content: url('http://www.sstscorp.in/images/QRIO.jpg.jpg');
}
.native-hidden {
    -webkit-appearance: none;
    -moz-appearance: none;
    appearance: none;
}
HTML:
<input class="native-hidden" type="checkbox" />
you can change the images in the css to your robot ones, input[type="checkbox"] will be your first image on page load and input[type="checkbox"]:checked will be the picture when its clicked.
http://jsfiddle.net/joshstevens19/goyenhLd/2/