Also, the creator of the stock imagery holds the copyright (unless it's specifically sold) and the images cannot be used or licensed or resold without their permission.
Royalty free (RF) can, in some cases, have unlimited usage given with the license purchase, so that a company could use it over and over again. RM (right management) can still be licensed for a small amount.
I wouldn't specifically say that RM puts limitations on that image's usage, especially not generally one year. It can be all over the place, and many companies don't want to pay enough to make it an exclusive usage, so other companies can continue to license it at the same time in a lot of cases. The difference is that they can also usually see a history of who has used it for what, so that McDonald's and Burger King don't end up using the same image at the same time.
Generally, the photos don't have re-use limitations set on them unless it's a very big license.
Stock is also kind of political now, because of the massive amounts available - license fees are always going dowwwwwn. Plus a lot of people are now using RF instead of the more traditional RM so that drives the market down as well. There's a ton of competition and the huge conglomerate, Getty Images, keeps buying up most of the market and driving prices down.
Plus there's a new thing called "Microstock", where images can be licensed for cheap cheap cheap (like $1 per image and up!) and sales are based on licensing images a bunch of times (rather than exclusively for a lot more $$). With the rise of consumer digital photography, things are swinging towards being much more widely available for less.
Sorry to go on and on - this is a huge issue in "the business" with much info written about it.