Though, it is not possible to create and customize an image, but only videos, you can create a video, and download it as image.
Once on the Publish Page, you'll see the “Export image/GIF" button, as on the screenshot.
Click on “Export as JPEG” button, choose any frame you need by moving the slider bar and click on “Export Photo” button.
You can download several images and upload them on FB to create a Facebook Slideshow Cover.
Note: You can add up to 5 images to FB cover slideshow.
Please note: For your text to look the best on your cover, make sure to place it in the center of the video. Facebook cuts off the edges of the cover.
Please check this article as well, to find out how to upload your images as Facebook Cover.
If you have any issues, just reach out to our 24/7 in app customer support team :)