okay well, First things first, You will have to change your works into books. This is how you do that. ( I suggest opening a tab so you can read this while doing it )
1. put your mouse over your name and click my works.
2. click "Create a New Story Group"
3. Put you title and the pages to the story and add a description. You will have to do separate ones for each of your stories.
Once you have your works into books you can now give it a cover and this is how.
1. If it is a book there should be a block that says "no cover" to the left of your title in the 'My Works' tab.
2. Click on the box that says 'No cover'
3. then it has a recommended pixel length, you don't have to follow it but it makes it look nicer in my opinion.
4. just upload the picture you want as a cover and there you have it. :)