You will need the Full Write Events permission or be an organizer on the event to perform this action.
One of the best ways to make your events standout is by adding a header image or graphic. This element not only adds interest to your event, but it also helps to convey important information.
Navigate to the event you want to upload/change the image for. Hover over the calendar graphic, and click on the pencil icon.
This allows you to upload an image from a URL, Unsplash, the Document Library or to upload a file from your hard drive.
Once uploaded, the image will appear under the "Info" tab. The recommended image size is 1600x500. However, if you upload an image smaller or larger, the system will resize as needed while maintaining the original aspect ratio.
Note, that for repeating events you can choose under "Actions" to update all events that are a part of this series or update only this event and keep other events in the series the same.