Change src pictures on data-src in field

template.php

function themename_preprocess_field(&$variables, $hook) {
  $element = &$variables['element'];
  if ($element['#entity_type'] == 'NAME'){

    foreach ($variables['items'] as $key => $val) {
     $output = $val['#markup'];
     $delta = str_replace('src', 'data-src', $output);
    };

    $variables['items'] = array(
        '#markup' => $delta,
    );

  };
};