Getty is expensive, but excellent.
Corbis is wonderful. As good as Getty, but you get what you pay for: low-rez are expensive too.
iStock is good and cheap. Low-rez
for $1.00. Can not beat the value. Not the quality of Getty or Corbis (yet), but for the price? Wow!
You can buya one-month subscription for US$129. Pretty good deal and quality images, though not as good as Getty.
Free clipart starting place
Links to myriad "free" sites. Huge site, but not all sites listed offer free images, so be careful.
A plethora of background images, and templates.