fleet wrote:I am not a lawyer.
Short answer, generally speaking, it's not legal.
It's funny how you give a definitive answer despite saying you're not a lawyer.
It's definitely not as clear cut as that it's always illegal, even if the photos are not explicitly licensed for public use. There's a legal grey area in fair use laws and derivative works, especially when it comes to so-called transformative use (re-using the original work in completely new ways). The boundaries are not very clear, so it's hard to say whether or not it's illegal until someone sues you and a court makes a decision on the legality. Google is your friend for finding out more about this stuff, but you probably won't find a clear cut answer.
There's some things to consider about the source material, of course. If the images are originally from a paysite, but made publicly available by some third party, your use is more likely to be illegal because your re-use may damage sales. If it's already available for free from the original publisher, for example as promotional material, then your re-use won't necessarily do any damage and thus be more likely to be "fair use". In either case, if you get these images from a third party, they are much more liable than you are.
Of course this all goes down the drain when you intend to make money off your game, either by sales or advertising..
But as an example, on TFGamesSite and the Hypnopics Collective there's a large amount of free games that use pictures found on the internet and I never heard of anyone getting into any problem because of that. Remember, we are just a small, obscure community, so the chance of anyone from those companies finding out is very small, while the sites where you're getting the images are much more public. This does not mean that it's strictly legal, but the chances of getting into trouble are incredibly small: if you don't do damage you're probably not worth suing (at worst you get a cease and desist warning). And even if they do, they have to prove that your use is outside any fair use laws. Bottom line, I personally wouldn't worry about it too much.