He estado tratando de resolver esto y hacerlo funcionar, pero como no soy un programador, solo un diseñador con conocimiento limitado de codificación, he estado golpeando una pared en este caso.

Estoy haciendo mi propia galería de fotos y me gustaría mostrar dos imágenes anteriores o siguientes como miniaturas vinculadas. Encontré este script que funciona y muestra enlaces textuales, pero no sé cómo adaptarlo a mis necesidades.

template.php

<?php
function dad_prev_next($current_node = NULL, $op = 'p') {
    if ($op == 'p') {
        $sql_op = '<';
        $order = 'DESC';
    } elseif ($op == 'n') {
        $sql_op = '>';
        $order = 'ASC';
    } else {
        return NULL;
    }

    $output = NULL;

    $sql = "SELECT n.nid, n.title
             FROM {node} n
             WHERE n.nid $sql_op :nid
             AND type IN ('photos')
             AND status = 1
             ORDER BY nid $order
             LIMIT 1";
    $result = db_query($sql, array(':nid' => $current_node -> nid));
    foreach ($result as $data) {
    }
    if (isset($data)) {            


        if ($op == 'n')            
            return l("Next", "node/$data->nid", array('html' => TRUE));
        else if ($op == 'p')
            return l("Previous", "node/$data->nid", array('html' => TRUE));
    }
}
?>

node.tpl.php

<?php print dad_prev_next($node,'p') . " / " . dad_prev_next($node,'n'); ?>

Ejemplo de trabajo

Esta muestra dos imágenes anteriores / siguientes tal como fueron diseñadas

template.php

function dad_prev_next($nid = NULL, $op = 'p', $start = 0) {
    if ($op == 'p') {
        $sql_op = '>';
        $order = 'ASC';
    }
    elseif ($op == 'n') {
        $sql_op = '<';
        $order = 'DESC';
    }
    else {
        return NULL;
    }

    $output = '';

    // your node must have an image type field
    // let's say it's name is IMAGEFIELD
    // select from node table
    $query = db_select('node', 'n');
    // join node table with image field table
    $query->leftJoin('field_data_field_photo', 'i', 'i.entity_id = n.nid');
    // join file managed table where all data about managed files stored
    $query->leftJoin('file_managed', 'f', 'f.fid = i.field_photo_fid');
    $query
        // select nid and title from node
        ->fields('n', array('nid', 'title'))
        // select uri from file_managed (image path)
        ->fields('f', array('uri'))
        // select image alt and title
        ->fields('i', array('field_photo_alt', 'field_photo_title'))
        // where nid "greater than"/"lower than" our current node nid
        ->condition('n.nid', $nid, $sql_op)
        // where node type in array('your content types')
        ->condition('n.type', array('photos'), 'IN')
        // where node is published
        ->condition('n.status', 1)
        // where requested node has image to display (if you want thumbnail)
        ->condition('f.uri', '', '!=')
        // order by nid
        ->orderBy('n.nid', $order)
        // limit result to 1
        ->range($start, 1);     

    // make query
    $result = $query->execute()->fetchAll();

    foreach ($result as $node) {
        // theme your thumbnail image
        $variables = array(
            // default image style name `thumbnail`
            // you can use your own by following
            // admin/config/media/image-styles on your site
            'style_name' => 'square_small',
            'path' => $node->uri,
            'alt' => $node->field_photo_alt,
            'title' => $node->field_photo_title
        );
        $image = theme('image_style', $variables);

        $options = array(
            'html' => TRUE,
            'attributes' => array(
                'title' => $node->title
            )
        );
        $output = l($image, "node/{$node->nid}", $options);
    }

    return $output;
}

node.tpl.php

<?php print dad_prev_next($node->nid, 'p', 1); ?>
<?php print dad_prev_next($node->nid, 'p', 0); ?>
<?php print dad_prev_next($node->nid, 'n', 0); ?>
<?php print dad_prev_next($node->nid, 'n', 1); ?>