Resizing and showing images from ACF in WordPress theme development for page load performance optimisation

functions.php

<?php
add_action( 'after_setup_theme', 'custom_image_sizes' );
function custom_image_sizes() {
    add_image_size( 'image-1400px-wide', 1400 ); // 1400 pixels wide (and unlimited height)
    add_image_size( 'image-700px-wide', 700 ); // 700 pixels wide (and unlimited height)
    add_image_size( 'image-350px-wide', 350 ); // 350 pixels wide (and unlimited height)
}

theme-template.php

<?php $image_src = wp_get_attachment_image_src( get_field('image_ID'), 'image-700px-wide' ); ?>

<!-- THIS WAY THE USER IS FREE TO UPLOAD AN IMAGE OF ANY SIZE (e.g.: from the smartphone) -->
<img src="<?php echo $image[0]; ?>" class="img-responsive product-image" />