You can easily customize Gallery cover images independently of your Collection and other published Galleries from your Gallery Settings!
To access the Settings of a Gallery you've already published, click Galleries & Apps in the top-right corner of your Collection and click Settings.
Hover the Gallery cover image and click to upload a new image:
Adjust the cover focal point, then save your changes!
If you have any questions or need additional help, feel free to contact us in the chat bubble in the lower right-hand corner of your dashboard. We're always happy to help!