nzbwolf (Open Registration)

This site also offers many porn nzb files with lots of preview images - they have good preview image coverage for porn content. Grab nzb files from nzbwolf.com
What kind of stuff has nzbwolf.com to offer: Lots of 0-day porn with screenshots and preview images from your favorite porn sites and studios. To just name a few: FakeTaxi, BlackedRaw, Blacked... Good thing is you can filter by quality and there are at least 20 new UHD / 4K releases each day.

nzbhydra (Open Registration)

This site offers many porn nzb files with lots of preview images - no other nzb indexers (accept kleverig which is a forum-based approach) comes close to that many preview image coverage for porn content. Grab nzb files from nzbhydra.com

kleverig (closed)

kleverig is a very active forum community dedicated exclusively to adult content that can be found on usenet. They have spotters and search for content on usenet. It is not an indexers but a forum with nzb files and lots of preview material. The site has recently been closed for new registrations. They have and will not do any kind of invite system. So you need an account from the past to get in there. If you ask yourself how to get invites to kleverig: not going to happen.