I would say somewhere between lines 588 and 599 would be where to show the picture associated with the product. At least this is the location I would suggest gather which picture should be displayed and then if to be shown elsewhere (bottom of the page or on a separate location), then so be it. But it is here where the product is individually stepped through and the attributes of the product are "collected". If the image is dependent onthe attributes then need to consider determination as a part of stepping through the attributes, otherwise would want to have the image information only as a part of the loop for the products in the cart...
Beyond that, yeah would want to consider if it is something to be included in the packing list or the invoice depending on how you do business.