Leanpub is mostly used by self-published authors who design and upload their own book covers, and add images to their books themselves. Authors are supposed to have the rights to any images they use.
If you want to report an infringement, and you want to follow our DMCA takedown process (at https://leanpub.com/takedown), please email us to this effect at hello@leanpub.com.
You can read about our takedown policy here: https://leanpub.com/takedown.
In the meantime, if you want us to contact the author of the book and pass your information along, please let us know.