Show one image on a node teaser

Hook_process_field can be used inside template.php to render only one image on a multiple image field.

 * Implements hook_process_field().
function MYMODULE_process_field(&$vars) {

  switch($vars['element']['#view_mode']) {
    // Only on node teaser.
    case 'node_teaser':
      switch($vars['element']['#field_type']) {
        // Only on image fields.
        case 'image':
          // Only render the first image of a multiple image field.
          $vars['items'] = array(reset($vars['items']));