Hi there Marie,
Thanks for getting in touch with us! I’ll help you out on this, although we cannot help much on theme customization.
I’d start by setting the calendar default view to ‘photo view’. You can hide some/all event details (title, description, etc) via CSS and just leave the picture. One that’s done you can insert the next upcoming event before the grid is shown. Do you think something like this would work?
One more thing if you are not planning to make this customization in the calendar page, you can use the tribe_get_events function to query events and just output their featured image if any. This article explains how that works → Using tribe_get_events.
Please let me know about it,
Best,
Nico