feiperinaldi, I solved this problem easily.
First, make sure the form on line 57 looks like this (note the enctype attribute)
<form id="event-form" method="post" action="" enctype='multipart/form-data'>
Paste the following code at line 324 of /events-manager/templates/event-form.php:
<div id="event-image" class="stuffbox">
<h3>
<?php _e ( 'Event image', 'dbem' ); ?>
</h3>
<div class="inside" style="padding:10px;">
<?php if ($EM_Event->get_image_url() != '') : ?>
<img src='<?php echo $EM_Event->image_url; ?>' alt='<?php echo $EM_Event->name ?>'/>
<?php else : ?>
<?php _e('No image uploaded for this event yet', 'dbem') ?>
<?php endif; ?>
<br /><br />
<label for='event_image'><?php _e('Upload/change picture', 'dbem') ?></label> <input id='event-image' name='event_image' id='event_image' type='file' size='40' />
</div>
</div>