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We can have a better solution, without override or modify PaymentModule.php

so Mail.php send an Array for product. We only 6 keys, but we have the id product. It's alll we need for the tpl;

smarty var for url img cannot work because product in Mail is a simple Array, but we not need img with the url rewrite inside an email. 

and in htaccess we can find line like this

RewriteRule ^([0-9])([0-9])([0-9])([0-9])([0-9])([0-9])(\-[_a-zA-Z0-9-]*)?(-[0-9]+)?/.+\.jpg$ %{ENV:REWRITEBASE}img/p/$1/$2/$3/$4/$5/$6/$1$2$3$4$5$6$7$8.jpg [L]

order_conf_product_list.tpl 

{foreach $list as $product}
     {assign var='defaultImage' value=Product::getCover($product['id_product'])}   // we have now the ID image

	 //// tr, td etc....
	<img src="DOMAINNAME/{$defaultImage['id_image']}-medium_THEMENAME/{$defaultImage['id_image']}-medium_default.jpg"  width="110" height="120"/>	

	//// replace DOMAINENAME by https://www.yourdomaine
	//// replace THEMENAME by your name theme

	//// tr, td etc....
{/foreach}

		

this code work because all image name is  related to id and the rewrite inside default htaccess

and if you look in an id in ftp you see for id 40  :  in ftp => p/4/0   : 40-small_default.jpg, 40-medium_default.jpg,40-large_default.jpg, etc......