Answered Thumbnail output filter - based on image class

<?php
function simpleStr($matches) {
  return $matches[0];
}

function encodeImg($matches) {
  $img = $matches[0];

  $cnt=preg_match_all('#src="[^"]*"#', $img, $srcarr);
  if ($cnt>0)
  {
    $src = substr($srcarr[0][0],5,-1);
    $thumb = "<a href=\"$src\" title=\"a bigger image\"  rel=\"lightbox-1\"><img src=\"[[phpthumbof? &input=`$src` &options=`&w=120&h=120&zc=1&fltr[]=gray`]]\" /></a>";
    return $thumb;
  }
  else 
  {
    return $img;
  }
}

$gallery = $modx->getService('gallery','Gallery',$modx->getOption('gallery.core_path',null,$modx->getOption('core_path').'components/gallery/').'model/gallery/',$scriptProperties);
if (!($gallery instanceof Gallery)) return '';


$output = $modx->resource->content;

$output = preg_replace_callback('#<img class="thumb" src="[^"]*" .*/>#', 'encodeImg', $output);


$modx->resource->content= $output;

return;