Haven't found anything on this matter on the forum so I just thought I'll give my input here on how to add thumbnails display to the Category Event Module (Upcoming Events module).
Just add this bunch of code into your template_name/html/mod_eb_events/default.php:
Code:
<?php if ($row->thumb && file_exists(JPATH_ROOT.'/media/com_eventbooking/images/thumbs/'.$row->thumb)) { ?>
<img src="<?php echo JURI::base().'media/com_eventbooking/images/thumbs/'.$row->thumb; ?>" class="eb_event_thumb"/></a>
<?php } ?>
