Koha currently uses OpenLibrary's "Legacy Book API" [1] to fetch book covers which "is a legacy endpoint and may be phased out in the future". Also, it's inefficient. The replacement seems to be the dedicated "Covers API" [2]. One drawback, though, it's currently rate limited to 100 requests per IP per five minutes. That might quickly be exhausted on the OPAC search result page. Thanks [1] https://openlibrary.org/dev/docs/api/books [2] https://openlibrary.org/dev/docs/api/covers