A while back I installed a forum on our webserver for intercompany use. Then I found, on the same forum site, a free package of avatars and I downloaded and installed them. I never really looked at the avatars, it was like a pack of 10,000 or something. The forum was lightly used by employees for a while, but aside from a few departments, doesn't really get used much any more. About a year ago I get a letter from some legal firm who is suing us for copyright infringement. They want $5,000 per image to drop the case.
See, while the forum was private, the directory that the files got stored in was not locked down, so it was technically possible for someone to find those images from outside our network. The company owns a ton of copyrighted images (mostly random looking photos they bought from the photographer). They have a scan engine that scours the internet looking for those images or even partials of those images, and they found a few in that directory on our webserver. The images in question were just random photos of a beach or a sunset or something.
I blew it off as a scam, but as a precaution I sent it on to our lawyer. He agreed with me but just to put it to rest, he sent a letter saying that A) the images in question were unintentionally stored on our server and we didn't know they were copyrighted, B) the images in question were never used by any employee, for personal or business use, and C) the moment we were made aware that there might be a copyright issue, the images were removed.
A year later, they are still pursuing it, and it has cost us money to continue to have the lawyer go rounds with these guys.
Bottom line: If someone has the resources and sees an opportunity, even if you have not used it for any kind of revenue stream, they WILL come after you for money. Whether they can win in court or not is irrelevant, they know as well as you do that if they file the papers, it will cost you money to have a lawyer respond, and they bank on you deciding to cut your losses and pay them a fraction of what you would pay your lawyer to fight them.
I don't agree with it, but I can vouch for the fact that if it is copyrighted and you are using it without permission or without paying royalties, you are opening yourself up to a liability.