Add a "Link to Link field" option to image fields. Requires the field_formatter_settings module.

link_to_link_field.php

<?php

define('FEATURE_SITE_LINK_FIELD', 'link_field');

/**
 * Implements hook_field_formatter_info_alter().
 */
function feature_site_field_formatter_info_alter(&$info){
  $info['image']['settings'] += array('image_link_field' => '');
}

/**
 * Implements hook_field_formatter_settings_form_alter().
 */
function feature_site_field_formatter_settings_form_alter(&$settings_form, &$context){
  $display = $context['instance']['display'][$context['view_mode']];
  $settings = $display['settings'];

  // Only build this information for image displays
  if ($display['type'] !== 'image') {
    return NULL;
  }

  $settings_form['image_link']['#options'][FEATURE_SITE_LINK_FIELD] = 'Link field';

  $field_name = $context['field']['field_name'];
  $entity_type = $context['instance']['entity_type'];
  $bundle = $context['instance']['bundle'];
  $instances = field_info_instances($entity_type, $bundle);

  $link_fields = array();
  foreach($instances as $key => $instance) {
    if ($instance['widget']['module'] == 'link') {
      $link_fields[$key] = $instance['label'];
    }
  }

  $settings_form['image_link_field'] = array(
    '#type' => 'select',
    '#title' => t('Link field'),
    '#options' => $link_fields,
    '#states' => array(
      'visible' => array('select[name="fields[' . $field_name . '][settings_edit_form][settings][image_link]"]' => array('value' => FEATURE_SITE_LINK_FIELD)),
    ),
  );
}

/**
 * Implements hook_field_formatter_settings_summary_alter().
 */
function feature_site_field_formatter_settings_summary_alter(&$summary, array $context){
  $display = $context['instance']['display'][$context['view_mode']];
  $settings = $display['settings'];

  // Only build this information for image displays
  if($display['type'] !== 'image') {
    return NULL;
  }

  if(!empty($summary)) {
    $summary .= '<br />';
  }
  if(!empty($settings['image_link']) && !empty($settings['image_link_field'])) {
    if($settings['image_link'] == FEATURE_SITE_LINK_FIELD) {
      $summary .= t('Linked to field: <em>%field</em>', array('%field' => $settings['image_link_field']));
    }
  }
}

/**
 * Implements hook_field_attach_view_alter().
 */
function feature_site_field_attach_view_alter(&$output, $context){
  foreach(element_children($output) as $field_name) {
    $element = $output[$field_name];
    $view_mode = $context['view_mode'];

    // Only work on image fields with the iamge formatter
    if($element['#field_type'] == 'image' && $element['#formatter'] == 'image') {
      $type = FALSE;

      $field_info = field_info_instance($element['#entity_type'], $field_name, $element['#bundle']);
      if(isset($field_info['display'][$view_mode])) {
        $formatter_settings = $field_info['display'][$view_mode]['settings'];

        if($formatter_settings['image_link'] != FEATURE_SITE_LINK_FIELD) {
          return;
        }
        $link_field_name = $formatter_settings['image_link_field'];
        if(!$link_field_name) {
          return;
        }
        $field_items = field_get_items($element['#entity_type'], $context['entity'], $link_field_name);
        $uri = $field_items[0]['url'];
        $attributes = $field_items[0]['attributes'];

        foreach(element_children($element) as $key) {
          $output[$field_name][$key]['#path']['path'] = $uri;
          $output[$field_name][$key]['#path']['options'] = array('attributes' => $attributes);
        };
      }
    }
  }
}