Change the link around images files in Omeka to point to the fullsize image, not the archival image.

link_images_to_fullsize.php

<?php

/**
 * Change the link around images files in Omeka to point to the
 * fullsize image, not the archival image.
 *
 * Uses the 'display_file' filter in Omeka 1.5.
 */
function link_images_to_fullsize($html, $file, $callback, $options, $wrapperAttributes) {

  // If the file has a fullsize derivatrive.
  if ($file->hasFullsize()) {
    $helper = new Omeka_View_Helper_Media;

    // Set the linkAttribute 'href' option to file_display_uri for the $file.
    $options['linkAttributes']['href'] = file_display_uri($file);

    // Sets the $html to the Omeka_View_Helper_Media::image method.
    $html = $helper->image($file, $options);

    // Our $html still doesn't have the wrapper, so we need to check for that
    // here, and wrap $html accordingly.
    $wrapper = !empty($wrapperAttributes) ? '<div ' . _tag_attributes($wrapperAttributes) . '>' : ''; 
    $html = !empty($wrapper) ? $wrapper . $html . "</div>" : $html;

  }
  return $html;
}

add_filter('display_file', 'link_images_to_fullsize');