The image and title of your custom tab can be edited under the Edit Card menu.
Begin by selecting your name at the top-right of the screen followed by Edit Card. Under the Edit Card menu, select the category labeled Premium Card Features.
From there, you have two ways in which you can customize your tab. The first option is to name your premium tab. In the field labeled Name Your Premium Tab, delete "custom" and enter your own title.
The second option is to choose your Premium Tab icon. Click any icon to add it to your tab.
Your custom tab is now finished, and you're ready to add the content!