Ok, I hadn't sufficient grasp on the way it works. First of all, my first mistake was to call the Itemloader with wrong attribute: Item instead of Item(). `l = ItemLoadel(item=Item(), response=response)` I forgot the parenthesis !
In second place, I misunderstood the way it works. The ItemLoader is just here to populate the Item. Once it's done, when the parse method return the item populated with the l.load_item() method, it's time for the pipeline you've registered in the settings to do its trick, and so, to populate the item.images attribute, download files, et cetera.