Fix cover format

[Adrián]

The current cover has a white background as it has been directly retrieved from the promotional image.

Here is the image with a good format (just the book cover, without background): https://i.imgur.com/W19ebUP.png

The source of the cover is the publishers' site: https://contracultura.cc/marx-xxi-ind...

[Sandra]

Done.

Also added all the authors. It will take a little while to show correctly, due to caching.