While you cannot upload a custom background for your event page at this time, you can upload a custom thumbnail photo to show on your event page by following these steps.
Step 1: From the Event List on the Scheduled Events page, click on the title of the event.
Step 2: In the Event Settings area click on the link 'Event Photo'.
Step 3: Choose a stock photo, or upload a new photo from your computer.
Photo Upload Requirements
Photos must be no more than 125 pixels wide x 200 pixels tall. You must already have your photo edited to the right size before uploading. You can use Pixlr Express, or another photo editing program, to edit your photos.
Only JPG or GIF file formats are accepted.
Photos must be no more than 200 Kb in size.