The Featured image for an event is a 1000x400 image that can be displayed on the site homepage in the event slider. Since it is displaying on the main landing page for the portal, the decision to feature an event in this slider is made by the portal owners. If you would like to have your event featured in this way then please reach out to the managers of the portal site to request that they feature your event.
Below is an example of a "Harvest Days" event featured image being featured on a portal home page. On most portal sites, there may be 5-20 images in the slider at any time. The events will be featured in chronological order.
How to feature an event (Portal users only)
A star icon will appear next to published events. A portal user can click that star to feature or stop featuring an event. Use the left-side navigation menu to go to the Events list. The star next to the publish toggle will be gray when the event is not featured and the star will be blue when the event is featured. To confirm, simply navigate to the homepage of the portal and verify that the event appears where it should based on the event date.
If the event is a recurring event (one event with multiple instances) then clicking on the star in the event list will feature all instances of the event. To feature individual instances of a recurring, you will need to navigate to the event's edit page and use the stars in the "Event Dates" card under the event name card.