I just usually do something along these lines....
<?php
$thumb = $page->cropimage->getThumb('thumbnail');
echo "<img src='{$thumb}' alt='{$page->cropimage->description}'>";
?>
It's been debated a few times before.
https://processwire.com/talk/topic/6468-symphony-forum-and-pw/
https://processwire.com/talk/topic/3536-forum-integration-module/
https://processwire.com/talk/topic/572-release-discussions/
I'm currently working on a simple forum based on comments, but it's very basic and isn't a module, so it's not something I'll be releasing etc, it just fits in with my site.
If you want a forum like this one, then you'll definitely be better off using dedicated software as suggested. But for a simple one, I'd advise to have a go yourself, as you will learn a lot. I've learnt so much since joining up here, I keep rewriting my code as there are always more efficient ways.
GuruMeditation's post in how could i delete a page instantly, without moving to Trash was marked as the answer