Well one thing you can do is to start a webnews service that index posts by group/genre. This can include images and videos for almost any type of porn you're looking to serve.
Due to the nature of the beast usenet is mostly safe. Yes it does contain material that is copyrighted. However this material is not posted by yourself, you are simply the intermediary (the storage system). Usenet has thousands of servers and since they share data (feeds) you have no control of inbound data.
A typical newsfeed is around 1TB per day. Out of that, there is probably 400GB per day of porn posted daily (maybe more). You could use this data on a server by purchasing a partial feed and using one of the many webnews daemons that are around. Many of those daemons are free, most run on linux only. If you worked it out nicely you could have 5GB of "fresh" porn daily and have old articles expire in 20 days. This would net you a 100GB spool which is not unreasonable considering the servers you can rent nowadays.
Heck you could do even less than that. 100GB of porn is a LOT. As an example feeds can be had for between 70 and 150 per month for 5GB per day (prices vary by provider).
Before everyone goes off and tells me and you that it's not legal I'll point out numerous cases involving usenet. You'll probably go off and search and find harlan ellison vs aol and the only reason aol lost is because they didn't remove the offending material.
Also remember the DMCA which allows protections for ISP's (this includes NSP's). This protection basically means if someone requests materials removed from your server then you remove them in a timely manner. However they must also prove that they own the copyright to that material.
In regards to the question above, "how is every picture copyrighted". In Canada, copyright is given to the creator when the material is created. This includes images and text. I do not know US law so I can't give you an answer on that (see below). So a personal picture is still copywritten by someone and if you don't have their permission it's not allowed to be used.
I am NOT a lawyer, and I am not a legal expert in any way. I am only stating my opinion and opinions about items learned in my years as a usenet professional. Seek legal advice from a competant lawyer before venturing into areas that you do not know about or are unsure about.
Good luck