Hey @mnussbaum,
Thanks for reaching out!
When it comes to setting a default featured image, there is unfortunately no way to do this with our plugins. You could conceivably do it with some extensive custom coding, but that is outside the scope of support here and so it may not be a good option.
An alternative way around this would be to turn not to the plugins, but to your theme, and find where it displays featured images.
In these instances, you can check if the event has a featured image, and if not, supply a backup image. This too will take some custom coding, so reach out to a professional developer if you need help with writing custom code, but a general walk through of this sort of customization can be found here → http://www.wpbeginner.com/wp-themes/how-to-set-a-default-fallback-image-for-wordpress-post-thumbnails/
I hope this information helps!
George