While I endeavor to use my own images as often as possible, there are times when it becomes necessary to use stock imagery. Here are the three primary websites I use for stock images:

Stock.xchg ~ www.sxc.hu
Royalty-free images. Licensing rights vary.

iStockphoto ~ www.istockphoto.com
Images for purchase. Various licenses available.

Fotolia ~ www.fotolia.com
Images for purchase. Various licenses available.