I haven't posted on low cost online photography sources for awhile. One of the newer ones is SnapVillage, a microstock photography company owned by privately held Corbis. They allow the "amateur" photographer to upload images, select their selling price from $1 - $50 and go to town. The site allows you to search by price, recency, snappyness and relevance. Here's a fact sheet on the site.
Another photo resource that I've come across is PhotoShelter. They also offer a marketplace, although their pricing is significantly higher than SnapVillage. On a less costly note you can buy web quality photos at UK-based PictureNation on a subscription or per image basis.
I'll be adding SnapVillage to version 8 of my resource listing for presenters. If you have other photography or presentation related resources that you feel should be part of this resource listing send them to me.
Note: this is one of my photographs taken at the Bean in Chicago last summer.