You need to check the page where the image is actually on and see how it is actually licensed there. You might not be allowed to use it, you might have to attribute the image to its creator or you might be able to freely use it (among many other possibilities). Stealing a John Doe's image from their blog or whatever website you are pilfering might never be noticed but unless you are complying with whatever license is outlined there (and if there is no explicit mention of a license I would assume you are not allowed to use them), you are still doing something wrong. And although the likelihood to get caught is small in such a situation, sometimes it happens and then your reputation will take a hit it does not need to take.
If it is some commercial endeavor from which you try to steal images then it is pretty safe to assume they will find out unless it never leaves your local hard disk. Any free samples you see will probably contain an invisible watermark which will not be easy to get rid off and some webcrawler somewhere will detect it.

Don't use anything you don't have permission to use. Ask yourself this "How good would it look on my portfolio if a potential employer notices I used stolen art?" There are places you can acquire game art and things listed in the public domain are OK. Everything else is copywrited. Better yet, how about contacting an artist and collaborating? That would look much better in your portfolio anyways.

When I say "watermark" I don't refer to an obvious human visible watermark. There are ways to embed a watermark in an image that is not only completely invisible to the human eye but will also survive a lot of image modifications. Unless you are applying something like a really aggressive cartoon shader to an image the assumption that a digital watermark will still be found in your image is a rather safe one.

You don't have to hire. There are plenty of artists looking for portfolio work as well that I'm sure would do it for free. Ask around on places like Deviant Art. Also, a simple Google search will turn up places for game art. Like: http://opengameart.org/