We highly recommend you upload a very good quality image before you publish your event as this will dramatically impact your audience engagement.
From the ‘Create event’ or 'Edit event' page tap the "add photo" / camera icon. From there you will get the Upload Photo page. Tap 'Choose Photo'.
You can then select your event images for your phone photo library. You will be given the option to crop and center your image.
The last photo you upload will be the primary image on your event which is the first image shown on your web event page.
Don't worry, if you want to change your image you can delete an image at any time from your event dashboard and add a new one.