EDIT

function compare_image_sources($a, $b) {

/**

 * @todo Please document this function.

 * @see http://drupal.org/node/1354

 */

function theme_cdm_description_element_image_source($variables) {

  $image_sources = $variables['image_sources'];

  $asListElement = $variables['asListElement'];

  $out = '';

  $separator = ',';

  RenderHints::pushToRenderStack('descriptionElementImageSource');

  RenderHints::setFootnoteListKey(UUID_IMAGE_SOURCES);

  // Sorting the image sources.

  usort($image_sources, 'compare_image_sources');

  // Generate the footnotes.

  foreach ($image_sources as $image_source) {

    $footNoteKeys = cdm_annotations_as_footnotekeys($image_source);

    foreach ($image_source->sources as $source) {

      if (_is_original_source_type($source)) {

        $fn_key = FootnoteManager::addNewFootnote(original_source_footnote_list_key(), theme('cdm_OriginalSource', array(

          'source' => $source,

          'doLink' => FALSE,

        )));

        // Ensure uniqueness of the footnote keys.

        cdm_add_footnote_to_array($footNoteKeys, $fn_key);

      }

    }

    // Sort and render footnote keys.

    $footnote_key_list_str = '';

    asort($footNoteKeys);

    foreach ($footNoteKeys as $footNoteKey) {

      $footnote_key_list_str .= theme('cdm_footnote_key', array('footnoteKey' => $footNoteKey, 'separator' => ($footnote_key_list_str ? $separator : '')));

    }

    // Return value!

    $out .= '<span class="descriptionElement descriptionElement-' . $image_source->uuid . '">' . $image_source->multilanguageText_L10n->text . $footnote_key_list_str . '; </span>';

  }

  RenderHints::popFromRenderStack();

  return $out;

}