There are many options out there, but keep in mind that the best sites, such as Getty Images, are expensive with low-rez images (suitable for slides or web) often $75-$100 depending on usage. Getty has great quality and selection, no question, but what if you can not afford to make a slide presentation costing hundreds or thousands of dollars in stock image fees? In this case, low cost royalty-free images are an alternative. Photo.com has a good monthly, flat fee service that is affordable at $139 per month. But the site I recommend most is iStockphoto.com.
About a year ago I mentioned to Guy Kawasaki that I had started a new design group in Japan. Guy then told me about his buddy's new organization called istockphoto.com and thought designers here in Japan whom I know might be interested. I was impressed with the site. At that time the selection at iStock was not huge by any means, but the search engine was good and I could find some gems in there. The best thing about iStock was the price — $1.00 per low-rez image and a maximum for the highest resolution images of $2.00 or $3.00 (U.S.). A fantastic price.
iStock is incredibly easy to use and after you search you can just roll over thumbs to get a larger view without having to open another page. You can even add a color field to your search. For example, I can modify my search on the fly for "Coffee AND Drink" by clicking a black square on the spectrum at the top of the page to narrow my search further to images featuring cups of coffee with black or dark backgrounds.
I do not suggest you limit your image searches to iStockphoto.com only — I have a shelf full of Photo CDs and subscribe to other photo sites as well myself — but this is one site that you really must bookmark. They are adding images every week and just keep getting better and better. iStock has a "free image of the week" so you may want to check back from time to time to see what's new (and free).
And just in case you were wondering, no, I do not know anyone at iStock...I'm just a fan.
Read iStockphoto's interview with Guy Kawasaki. Guy offers some good advice for designers and design agencies. And in this interview with Yorkshire Media, Guy answers an important question concerning the iStockphoto business model (half way through the interview).
Read what iStock is all about here (includes a movie of how it works).
If some of you have a favorite site for getting images, I'd love to hear about them.