Right after creating your cover, you'll find all formats under "Your cover is ready". There you can download everything as a ZIP or each format individually.
Without a contract or as a visitor on the website: The download link is currently valid for 60 minutes. Download your covers within this time. After that, you'll need to create the cover again. There is no permanent list, and you can only create a limited number of covers per hour.
As an artist with a contract: Every cover is saved permanently under "My saved covers". Click the image to view it in full size. "Download" downloads all formats as a ZIP, "Formats" shows the individual sizes for viewing and downloading. Use the trash icon to delete a cover from your list.