The image pulled from Facebook is pulled directly from your event card. Check out how to edit your event card with these steps.
Sometimes when you make changes to your page or event card, Facebook doesn't update right away.
To update your Facebook image to your recent changes, check out Facebook's debugger tool:
1. In the event, head to the Settings tab..
2. Click into the Sharing Options section.
3. Under Facebook Settings, scroll down.
4. Click on the Facebook Debugger's Tool link.
5. Click on the "Show existing scrape information" button.
6. Then click on the "Fetch new scrape information" button. (You may need to click this a couple of times).