var checked = new Image();
var unchecked = new Image();

checked.src = '/i/bron_checkbox.jpg';
unchecked.src = '/i/chair_occuped.jpg';

function SwitchImage() {
    if (document.getElementById('my_check').checked == true) {
        document.getElementById('my_check').checked = false;
        document['check_image'].src = unchecked.src;
    } else {
        document.getElementById('my_check').checked = true;
        document['check_image'].src = checked.src;
    }
}