Free is good. Here are a few thoughts on where to obtain free high-quality, high-resolution stock images and artwork.
Freerange Stock is an advertising-supported site that encourages photographers to upload their content, and ad revenue is shared with the photographers.
Flickr is the granddaddy of all photo-sharing sites. The site contains millions of images, ranging from really bad to really good. If you need an image for a commercial project, limit your search to images with a Creative Commons license. Instead of using Flickr's slow, cumbersome search engine, try Compfight, a blazingly fast search engine for Flickr images.
Shutterstock* claims to be the "largest subscription-based stock photo agency in the world." They offer a new "Free Photo of the Week" and "Free Vector of the Week" every week on their front page.
morgueFile is another popular collection of user-submitted photography with a clear license and a good variety of images.
* Full disclosure: I'm an affiliate of Shutterstock and Crestock. This means that if you visit either site from here, and eventually buy something, I might make a few bucks.