Had the same question thats why i entered this 'stack'
Challenge: Wanted image with different sizes then default product.image size in #layer_cart
Solution:
Made an image with name popup_default in the BO of prestashop.
In /themes/theme_name/modules/blockcart/blockcart-json.tpl added the following at line 36
"image_popup": {$link->getImageLink($product.link_rewrite, $product.id_image, 'popup_default')|json_encode},
Then in /themes/theme_name/js/modules/blockcart/ajax-cart.js
made the following change at line 737
$('.layer_cart_img').html('<img class="layer_cart_img img-responsive" src="' + product.image + '" alt="' + product.name + '" title="' + product.name + '" />');
into
$('.layer_cart_img').html('<img class="layer_cart_img img-responsive" src="' + product.image_popup + '" alt="' + product.name + '" title="' + product.name + '" />');
Now i use an image with 'popup' sizes