The image displayed on Twitter when your event URL is shared is pulled directly from your event page's share card. Check out how to edit your share card for a full overview on this functionality.
Typically, when you make any changes to share cards or sharing descriptions and then share the link right away, these changes are not immediately reflected. This means that if you change your event card design or fix a typo in the title, it could take a few days for the changes to appear based on the sharing platform's caching.
Looking to speed up the process? Check out the Twitter Card Validator tool to pull the latest version of your share card at any time.