As mentioned earlier the usual stock photography suspects are; istockphoto.com, shutterstock.com, dreamstime.com If you are looking for commercial collections there also other well known companies like: Corbis Images, Veer and Getty Images.
Look at what the licensing options are and compare them to what you want to achieve and to what your end users require. Rights-managed (RM) is often the best way to go because it gives you and your end user the most control over replication of the images and products you are creating.
There are two recent cases that are famous in advertising circles. In both cases a pair of advertisers used cheap, royalty free images in outdoor billboard campaigns. The advertisements looked great, unfortunately, the competing advertisers used the same RF photo to promote their products. Making matters worse, the billboards were located side by side on a major thoroughfare and tens of thousands of potential customers saw the screw up as they drove by. Ouch!
One of those companies was a client of mine and they now exclusively create custom photography for their campaigns so that they retain control of their brand. In Canada, custom photography is often as inexpensive as RM (rights-managed) stock photography and is certainly the better option since you get exactly what you want.
Read about a similar screw up during the recent British election.
Post by Brett Gilmour