trigger_change('render_element_description',
          $picture['current']['comment'],
          'picture_page_element_description'
          )

string nl2br ( string $string [, bool $is_xhtml = true ] )

function pwg_nl2br($string)
{
  // echo '['.__FUNCTION__.'] nb args = '.func_num_args().'<br>'; // => gives 2

  return nl2br($string);
}

[pierrick@pierrick-desktop] ~/public_html/piwigo/dev/trunk
$ svn diff picture.php 
Index: picture.php
===================================================================
--- picture.php (revision 29781)
+++ picture.php (working copy)
@@ -137,7 +137,11 @@
 // add default event handler for rendering element content
 add_event_handler('render_element_content', 'default_picture_content');
 // add default event handler for rendering element description
-add_event_handler('render_element_description', 'nl2br');
+add_event_handler('render_element_description', 'pwg_nl2br');
+function pwg_nl2br($string)
+{
+  return nl2br($string);
+}
 
 trigger_notify('loc_begin_picture');

add_event_handler('render_element_description', 'nl2br');

add_event_handler('render_element_description', 'pwg_nl2br');
function pwg_nl2br($string)
{
  return nl2br($string);
}