Sometimes when you decide to share your event on Facebook, Facebook may not have actively checked the page for your images or information.
This can be fixed by going to Facebook's Sharing Debugger page, from there click on the "Sharing Debugger" tab and enter the URL(The address found at the top of browser, for example "www.billetto.co.uk/e/1234") of your event and click the "Debug" button.
1. If your link is new, you might need to click "Fetch new information" button to get Facebook to recognise the link.
2. If the image still doesn't appear, click "Scrape again" button until the correct image appears.
Once this has completed when you next create a post you should see the updated text and images. Sadly this does not work retroactively with old posts.
To clarify: this happens with most online media & Facebook. Sometimes Facebook robots are not fast enough to keep up with media being published online & need a bit of a nudge.