Example of using custom thumbnails for Events Manager plugin for WordPress.

events-manager-custom-thumbnails.php

<?php
/*
Plugin Name: Events Manager custom thumbnails
Plugin URI: https://gist.github.com/webaware/8793362
Description: example of using custom thumbnails for events
Version: 1
Author: WebAware
Author URI: http://www.webaware.com.au/

@ref: http://snippets.webaware.com.au/snippets/stop-events-manager-from-cropping-thumbnails/
*/

add_filter('em_event_output_placeholder', 'events_manager_custom_thumbnails', 10, 3);

/**
* get event image as regular WordPress thumbnail
* (or any registered WordPress image size)
* @param string $result
* @param EM_Event $EM_Event
* @param string $placeholder
* @return string
*/
function events_manager_custom_thumbnails($result, $EM_Event, $placeholder) {
    // check a custom image placeholder, pick up size
    switch ($placeholder) {
        case '#_CUSTOMEVENTIMAGETHUMB':
            $size = 'thumb';
            break;
        case '#_CUSTOMEVENTIMAGEMEDIUM':
            $size = 'medium';
            break;
        default:
            $size = false;
    }

    // if size was set, get image of that size
    if ($size) {
        $imageID = get_post_thumbnail_id($EM_Event->post_id);
        if ($imageID) {
            $result = wp_get_attachment_image($imageID, $size);
        }
    }

    return $result;
}