Just when you think you have the hang of creating a Facebook event page, it changes again.
Gone are the days of the “poster” image that you would see in the top left corner (event pages created prior to the change will still retain this). Now, event pages have a new “Event Photo” banner on the top. When displaying the event thumbnail on your page, it only takes an even smaller portion of this (you can re-position the image). The new image banner is 714 x 264. Anything taller will be cropped (much like a Facebook milestone image). Images that are too narrow will be stretched…and no one likes distorted, stretched images.
The thumbnail preview will still be 111 x 74.
Finally, in addition to learning how to create a proper image for your event, you should also learn some best practices. For example, don’t invite everyone on your friends list. You’ll annoy them and they’ll block you from future invites. Instead, focus on the ones actually in the area. Here was a guide I created for bands to promote their shows properly but it translates to almost any kind of event.