On your Bookshelf page below My Subscriptions, click the Monthbook you want to edit.
On the right-side menu of your screen below Cover Options, select the Change Cover button.
Finally, choose a photo from the options displayed or Upload a photo from your computer, and the cover image of your Monthbook will change.
If you would like to change the cover image of your Monthbook on the Chatbooks app, click here.
Please keep in mind that if you have a designer cover chosen for your subscription, a photo can not be added.