Anyway, i figured it out.
Probably not in the most "p-r-o" way, but it's fine now.
I just cut out the contents of the product_thumbnail.tpl right into the related_products.tpl ( in the row where the thumbnail has to go) and changed the alt tag var from {$product|escape} to ="{$product_links[cat_num].product|escape}
Seems to work perfectly... But I don't know, I'm not one of those pros...
