When someone shares an event or calendar link on social media or in a chat app, the preview uses the event's or calendar's image. To change the preview, upload a different image on the event or calendar.
When there is no image
Premium and Business: the preview shows your account logo from Settings > General > Branding Settings. If you have not uploaded a logo, no preview image is shown. See Branding and design templates.
Free: the default AddCal image is used.
The old image still shows
Social networks and chat apps cache link previews, so a new image can take a while to appear on links that were already shared.