Great question. Basically, it depends on what your intended use. Generally a small license fee opens up use for most commercial work (advertising, corporate video, websites), with some limitations on the number of times a stock image can be reproduced (istockphoto.com’s standard license is under 500,000 uses). Items for resale, like taking a stock image - unaltered - and putting it on a t-shirt, require a larger license fee.
For creative or artistic purposes, things get a little murky thanks to the introduction of something called derivative works. A derivative work is something based on one or more pre-existing works (i.e. translations, art reproductions, etc.). Generally, if a derivative work shows some originality of its own, it becomes a new work, protected by its own copyright. So if I take a bunch of stock images, printed, cut them out, and glued them together in a collage, that would be a “new work”, and as the copyright holder of this new work, could sell it to my heart’s content. If I take a stock image, print it out, and glue it to a tile I have not transformed the original enough, so this would be the same as the t-shirt example above.
There is also something called fair-use that allows for the re-use of copyrighted material without credit or licensing. If you are interested about it you can read more here.
As great as all of this is, none of this will keep you from getting sued. Shepard Fairey is currently in a back-and-forth lawsuit with the Associated Press over his Obama poster. The AP took the original image which Fairey based his poster on, and Fairey did not cite or license the original photo in any fashion. I think the poster is a pretty solid example of a new, derivative work covered under the fair-use doctrine, but there are arguments that cut both ways.
Hope that helps, and if you have other questions, feel free to ask.