Use DC:Description on File record for alt attribute in Omeka 2.x.

use_file_description_alt.php

<?php

/**
 * Use the DC: Description field on a File record for an image's alt attribute. Add this snippet to
 * a plugin or to your theme's custom.php file.
 * 
 * See https://github.com/omeka/Omeka/blob/master/application/views/helpers/FileMarkup.php#L744-751
 * for parameters available to the file_markup filter.
 * 
 * See https://github.com/omeka/Omeka/blob/master/application/views/helpers/FileMarkup.php#L619-647
 * for details of what Omeka_View_Helper_FileMarkup::derivativeImage outputs.
 **/

function add_file_description_alt($html, $params) {

    if ($params['callback'] == 'derivativeImage') {

        // Set the alt attribute to DC:Description field on the File record.
        $params['options']['imgAttributes']['alt'] =  metadata($params['file'], array('Dublin Core', 'Description'), array('snippet' => 100));

        // Create a new FileMarkup helper to reconstruct our HTML.
        $helper = new Omeka_View_Helper_FileMarkup;

        // Specifically set HTML based on callback in Omeka_View_Helper_FileMarkup.
        $html = $helper->derivativeImage($params['file'], $params['options']);

        // Copy/paste wrapper from Omeka_View_Helper_FileMarkup::fileMarkup.
        $wrapperAttributes = $params['wrapper_attributes'];
        $wrapper = !empty($wrapperAttributes) ? '<div ' . tag_attributes($wrapperAttributes) . '>' : '';

        $html = !empty($wrapper) ? $wrapper . $html . "</div>" : $html;

    }

    return $html;

}


add_filter('file_markup', 'add_file_description_alt');