I recently learned of a new on line stock photo agency, http://www.istockphoto.com. They use a system of pre-purchased credits, either just enough for one image or a pack of credits you can use as you choose. The average price is the main advantage—$15.00 per image. Now, if, like me, you do projects where the ad is a small local black and white where you need some punch, you can create something eye-catching without blowing the budget.
I do a lot of jobs for clients, especially non-profits, who simply don't have budgets for much more than the media they are purchasing. While istockphoto's images suffer from the same problem most stock images do—trite concepts—they are reasonable enough that you can afford to use a chunk of one for an illustration base, or simply an accent.
Creative applications of images, say as parts of montages or blown up as a background or even just a visual field can lend a lean ad some heft. The resolutions available allow you to take part of an image and enlarge it until it is filling an average frame or covering a large ad.
And at the prices they are asking, istockphoto's images are within the reach of small design shops and students.