a.compare-toggle {
    background-image: url("/path_to_image/checkboxes.png");
    background-repeat: no-repeat;
    color: #000000;
    height: 16px;
    padding-left: 20px;
    text-decoration: none;
}
a.compare-toggle.add {
    background-position: 0 0;
}
a.compare-toggle.remove {
    background-position: 0 -16px;
}