Checkbox image replacement

HTML

<div class="checkbox_wrapper">
    <input type="checkbox" />
    <label></label>
</div>

CSS

.checkbox_wrapper{
	position: relative;
	height: 16px;
    width: 17px;
}

input[type="checkbox"] {
    opacity:0;
    height: 16px;
    width: 17px;
    position: absolute;
    top: 0;
    left: 0;
    z-index: 2;
}

input[type="checkbox"] + label{
    background-image:...