[HawaiianGecko]

Someone using your router does not give them access to your computer necessarily.  I see no reason on Earth why they would actually place pics on your machine unless they were using you machine as a server to the outside world.  Typically they would set up series of innocuous folders on your machine and then give that address out. 

Now unless you have a block of specific IP addresses assigned to you by your ISP, even this won't work.  Most DSL, cable hookups use DHCP and all routers that I know of, along with any firewall built in the last 10 years won't allow this to happen by default.

My bet is that someone is downloading this directly from your computer, so you might want to check around on your own before calling the police. 

[texas booster]

Be careful in assuming that your BIL is the guilty party.

We once had a (minimally) secured server compromised and used as a relay server by a guy in Brazil. Mostly copyrighted movies, little that was porno.

We only noticed it when our backup tapes took longer than usual to complete.

We had static IP addresses, but I have heard of cases where dynamic IP addresses, which are recycled every week or so, could also be used as relay servers.

One such case I personally worked on was the mom of an NFL player. After a week or so the pirates just moved on, until they found her new IP address. Its just a block of numbers that are shared by systems on the high speed network.

Its a dangerous world out there.