You can add a cover photo to any invite or poll by following these steps:
Create a new link or edit an existing link.
Under Basic Info click the photo icon next to Cover Photo.
A file select dialog will appear. Select a single image from your computer or mobile device to use as the cover photo.
An Edit Photo dialog will appear. Here you can zoom or crop the photo to fit the cover photo dimensions. Click Save when you're done.
Step through the wizard and Save your link.