HTML ficará assim:

  <label>
<input type=”radio” name=”fb” value=”small” />
<img src=”http://placehold.it/20×20/35d/fff&text=f”>
</label>

<label>
<input type=”radio” name=”fb” value=”big”/>
<img src=”http://placehold.it/40×60/35d/fff&text=f”>
</label>

 CSS ficará assim:

label > input{ /* HIDE RADIO */
display:none;
}
label > input + img{ /* IMAGE STYLES */
cursor:pointer;
border:2px solid transparent;
}
label > input:checked + img{ /* (CHECKED) IMAGE STYLES */
border:2px solid #f00;
}

Exemplo: http://jsbin.com/tosokawose/edit?html,css,output