You can add a custom text to the invitation email, by editing the field in the email preview, within the main screen, which you can reach from the “Invite by email” link in the menu on the right side of the event creation/edit page.

You can also replace the cover photo of the email invitation, and choose one that is better viewed in a smaller format. The cover picture is by the fault the same that you inserted for the event page, and this way you can enhance the recognizability of your event.