A good photo goes a long way when conveying a message. Luckily, more and more photo sites have turned up, where either all or some of the collection are completely free to use without attribution.
As designers and visual communicators, we need to make sure to use photos to help tell a story and to convey a message. While it's best to use high quality original photos whenever possible, sometimes they just won't fit into the workflow. Either you ran out of time, don't have access to the subject matter, or you don't have the equipment or patience to take a really great photo.