Drupal node image field links to other link field

template.php

<?php
/**
 * @file
 * Preprocessing functions for event nodes.
 */

/**
 * Implements hook_preprocess_node().
 */
function _[yourthemename]_preprocess_field_name__[nodetypename]__[imagefieldname](&$variables) {
  if (empty($variables['element']['#object']->linkfieldname['und'][0]['url'])) {
    return;
  }

  foreach ($variables['items'] as &$item) {
    $image = drupal_render($item);
    $item = array(
      '#type'=>'markup',
      '#markup'=>l($image, $variables['element']['#object']->linkfieldname['und'][0]['url'], array(
        'attributes' => array(
          'target' => '_blank',
        ),
        'html' => TRUE,
      )
      ),
    );
  }
}