title: Set custom image to checkbox without mandatory label
body: <p>I have a project where sometimes checkboxes don't have labels. The only way of setting custom image for checkbox I've found was setting image for <code>label:before</code> for corresponding <code>label</code> which has <code>for</code> value with <code>id</code> of checkbox.</p>
  
  <p>Are there any CSS way (at least hacky) to set custom image to checkbox without changing markup? <code>input[type="checkbox"]:before</code> works only in Chrome.</p>