you are a lucky boy, when you use ss2.3.1

http://doc.silverstripe.com/doku.php?id=simpletreedropdownfield

hm i looked into the DB tabel: FILE

i think all entries there are entries what are in "assets", sooo..

there are some different "ClassName" like

Folder
Image
File

###############################

when i want to make a page, that returns all my uploaded googleanalytics from the "googleanalyticsfolder" in my assets, i have to look up, what ID this folder has. then i use this code:

mysite/code/GoogleAnalytics.php
<?php

class GoogleAnalytics extends Page {
   static $db = array(
   );
   static $has_one = array(
   );
}

class GoogleAnalytics_Controller extends Page_Controller {
   function GoogleAnalyticsList() {
      
      $GoogleAnalyticsList = new DataObjectSet();
      
      $result = DB::query("SELECT * FROM File WHERE ParentID = '293' ORDER BY Name DESC");
      if($result) {
         foreach($result as $value) {
            $eintraege = array(
                           'ID' => $value['ID'],
                           'Name' => $value['Name'],
                           'Title' => $value['Title'],
                           'Filename' => $value['Filename']
                           );
            $GoogleAnalyticsList->push(new ArrayData($eintraege));
         }
      }
      return $GoogleAnalyticsList;
   }
}

?>

themes/MYTHEME/templates/Layout/GoogleAnalytics.ss

...
...
<center>
<% control GoogleAnalyticsList %>
<a href="$Filename" target="_blank">$Name</a>
<% if Last %><% else %><hr /><% end_if %>
<% end_control %>
</center>
...
...
###############################

so, back to problem:

you can get with that code all "Images" or all files except folders (ClassName != 'Folder'), or you can get at first all Folders with ID, then you can fill between the folders an array with the files, who has ParantID = current Folder ID, after this you can output it nice..

hm, but just for outputting all Images or all Files, you can use this code i think..

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+ I know, there is a better way with "dataobject::get(..." or stuff like this.. but i did not yet used it.. but soon ;) i promise!
+
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

i hope you need some stuff from here^^

I will give it a try. Thanks for that & greetings from Cologne!