One of the most important tips for a great event image can be removing or limiting the amount of text present in the cover photo. When seen from different devices with different screen sizes, it can be very hard for your guests to always see the text clearly. The other most important consideration for a great event image is the size. You will want to upload a picture that fills most of the page. You generally want to use an image that is much wider than it is tall. An aspect ratio of 2:1 is a good estimate.
TIP: Center your content on your image to give the best mobile experience.