I would think that usage falls under Fair Use.
- Not being used commercially or as part of the site disign.
- Small/low res so can't be reprinted
- You could probably claim educational/reporting/research purposes of use
- Apparently covers have copyright as part of the book, making the usage a very minor portion of the whole ( see #2 here: https://psu.libanswers.com/faq/336502 )