// questa funzione è da inserire nel file functions.php del tema
function print_your_theme_name_image( $image_data, $image_sizes ) {
  if( count( $image_data ) > 0 ) {
    $image_data_select = $image_data['image_type']; // post_thumbnail, acf_field or acf_sub_field
    $size_fallback = $image_data['size_fallback']; // crop to use as fallback
    if ( $image_data_select === 'acf_field' ) { // ACF FIELD - be sure to set "image -> image array in ACF options"
      $thumb_id_pre = get_field( $image_data['image_value'] ); // retrieve the image data array
      $thumb_id = $thumb_id_pre['ID']; // retrieve the image ID
    }
    elseif ( $image_data_select === 'acf_sub_field' ) { // ACF SUB FIELD - be sure to set "image -> image array in ACF options"
      $thumb_id_pre = get_sub_field( $image_data['image_value'] );
      $thumb_id = $thumb_id_pre['ID']; // retrieve the image ID
    }
    elseif ( $image_data_select === 'post_thumbnail' ) { // nomrmal featured image
      $post_id = get_the_ID(); // retrieve post/page ID
      $thumb_id = get_post_thumbnail_id( $post_id ); // retrieve the image ID
    }
    if ( $thumb_id != 0 ) {
      $attachment_title = get_the_title( $thumb_id ); // image title
      $attachment_alt = get_post_meta( $thumb_id, '_wp_attachment_image_alt', true ); // image alt text
      if( count( $image_sizes ) > 0 ) {
        $print_theme_image_cache_key = 'print_theme_image_cache_'.$thumb_id;
        $sharped_images = wp_cache_get( $print_theme_image_cache_key ); // check if array of images URL is set as cache
        if ( false === $sharped_images ) {
          $sharped_images = array(); // declare arry to use later in data-srcset
          foreach( $image_sizes as $image_size) {
            $thumb_url[$image_size] = wp_get_attachment_image_src( $thumb_id, $image_size, true ); // retrive array for desired size
            if ( $thumb_url[$image_size][3] == true ) { // check if cropped image exists
              $sharped_images[] = $thumb_url[$image_size][0]; // retrive image URL
            }
            else { // if cropped image does not exist
              $thumb_url[$image_size] = wp_get_attachment_image_src( $thumb_id, $size_fallback, true ); // retrive array for fallback size
              $sharped_images[] = $thumb_url[$image_size][0]; // retrive fallback image URL
            }
          }
            wp_cache_set( $print_theme_image_cache_key, $sharped_images, 300 ); // set array of images URL as cache
        }
        $html_image_output = '';
        $html_image_output .= '<div class="no-the-100">';
        $html_image_output .= '<picture>';

        $html_image_output .= '<source media="(max-width: 1024px)" data-srcset="'.$sharped_images[2].', '.$sharped_images[3].' 2x">';
        $html_image_output .= '<source media="(min-width: 1025px)" data-srcset="'.$sharped_images[1].', '.$sharped_images[0].' 2x">';
        $html_image_output .= '<img data-src="'.$sharped_images[1].'" src="'.$sharped_images[4].'" title="'.$attachment_title.'" alt="'.$attachment_alt.'" class="lazy" />';
        $html_image_output .= '</picture>';
        $html_image_output .= '</div>';
        echo $html_image_output;
      }
    }
  }
}

// questo snippet invece richiama la funzione e può essere usato dovunque serva mostrare l'immagine
$image_data = array(
    'image_type' => 'post_thumbnail', // options: post_thumbnail, acf_field, acf_sub_field
    'image_value' => '', // se utilizzi un custom field indica qui il nome del campo
    'size_fallback' => 'full_desk'
    );
$image_sizes = array(
    // qui sono definiti i ritagli o dimensioni. Devono corrispondere per numero a quanto dedinfito nella funzione nei parametri data-srcset o srcset
    'desktop_retina' => 'full_column_desktop_retina',
    'desktop' => 'full_column_desktop',
    'mobile' => 'full_column_mobile',
    'mobile_retina' => 'full_column_mobile_retina',
    'micro' => 'micro'
    );
print_your_theme_name_image( $image_data, $image_sizes );