Sometimes you might have to specify an exact dimension to suit your needs. concrete5 has several helper classes, one called image which offers several functions, including a function which generates and caches a thumbnail.
<?php
defined('C5_EXECUTE') or die('Access Denied.');
$th = Loader::helper('text');
$ih = Loader::helper('image');
?>
<div class="ccm-page-list">
<?php
foreach ($pages as $page):
// Prepare data for each page being listed...
$title = $th->entities($page->getCollectionName());
$url = $nh->getLinkToCollection($page);
$target = ($page->getCollectionPointerExternalLink() != '' && $page->openCollectionPointerExternalLinkInNewWindow()) ? '_blank' : $page->getAttribute('nav_target');
$target = empty($target) ? '_self' : $target;
$description = $page->getCollectionDescription();
$description = $controller...