Add class to radio button

Add class to radio button

<input id="radio1" type="radio" name="rgroup" value="1" >
<label for="radio1"><span><span></span></span>1</label>
<input id="radio2" type="radio" name="rgroup" value="2" >
<label for="radio2"><span><span></span></span>1</label>
I have something like this and i want to add an image to the element which
is checked using javaScript. How can i do it?