Shutterstock is one of my favorites. They have great images and for $249 per month you can upload 25 images a day for a total of 750 images per month. I've done it a couple times for just one month and have a large library of images now. Quality is excellent. Always upload the largest "free" image.
This one is associated with stockexchange but its got free images on it: http://www.sxc.hu/index.phtml I've found some interesting things on it so far.