Suppose i want to implement a responsive header with compliance for multiple devices. To do so (with a beautiful result ofc), i need different versions of the picture, resized and cropped by their sides, then put all of them in a … block.
I could just upload multiple versions of it, but that wouldn’t be user friendly, since i’m forced to create the pictures one by one.

I thought something like adding

add_image_size("grand_neo", 320, 450, true);

to functions.php, then in my header.php get it with

wp_get_attachment_image($id, 'grand-neo');

where $id is the header image’s one. But how to get it?
Thanks anyone willing to help.

Read more here: Automatically generate multiple image sizes for <picture> element