Hi alonso,
Thanks for using our plugin.
I can help you get started on this and then you should be able to take it from there.
You are going to want to follow our Themer’s Guide for this customization:
https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/
For the Single Events you want to find this file and move it to your theme according to the themer’s guide”
\the-events-calendar\views\single-event.php
Then look for this line:
<?php echo tribe_event_featured_image( $event_id, ‘full’, false ); ?>
That is what you will have to modify to add a default image.
If you get stuck let us know, but for the most part this customization is beyond the support we can give on the forum from here.
Thanks