Show Thumb Image When User Press Add To Compare

Hi everyone
i want to show thumb image of each product that user choose to compare in the page
currently just green box appear in top section of the page and show a text message that for example sony vaio successfully added to compare list , i want to show thumb image of the sony vaio in the green box
i found that in the catalog\controller\product\compare near the end of the page we can define $json['variable'] and in the \catalog\view\javascript\common.js near the --> function addToCompare(product_id) --< this function call json['variable']

but i dont know how to save thumb url path in the json in the compare.php

thanks