By now everyone knows that using professional images in your presentation is far better than ripping images from Google image search or clipart: higher quality photographs, isolated subjects on a white background, detailed search capabilities including required colors or available white space for type, and last but not least: no copy right infringement issues.
With the increase in popularity of stock images also came a backlash: many photographs were so cliche and/or over-used that designers increasingly start to look at other image sources with creative common licenses (I like Flickr a lot).
Price is another issue. Online stock image sites used to charge around $1 for each image. At that price you could afford to buy volumes and volumes of images, try them and discard them if they were not appropriate. Prices have gone up significantly recently, requiring a change in the creative process: design your presentation with low-resolution comps and only buy your images at the very last stage of the project.
Technology is about to put new power in the hands of stock image buyers. Many stock image sites contain the exact same image, but offer them at different prices. Differences in price are the result of general pricing policies (driven by the strength of the brand of the stock image site) or sophisticated dynamic pricing algorithms, setting image prices based on the number of downloads/views (more popular images become more expensive).
SpiderPic is a price comparison search engine for stock images and let's you decide from which source you want to buy the image. You key in the search term, the site presents the available options, and once you select a candidate it lists other sites that offer the image (and at what price for what resolution). Once you made your selection you are linked through to the relevant stock image site to complete the purchase transaction.

The site is currently in private beta, but the home page states that they are considering applications from people who want to give it a try.