Without Events Calendar Pro
The Events Calendar ships with a simple, but neat widget called Events List which makes it easy to add a list of upcoming events to your sidebar.
By default, Featured Images are not displayed with each event, but you can add this functionality by pasting the following snippet into your theme's functions.php file. It's as simple as that! :-)
With Events Calendar Pro
If you're using Events Calendar Pro, the above function may not work well for you. Instead, you'll likely have better results by making a new template file for The Events Calendar altogether. To learn about making template files for The Events Calendar, check out our comprehensive Themer's Guide here.
An example of how to make a template that has thumbnails, check out: