The very best place to start when you are looking for images is Creative Commons. They provide a way to search for Creative Commons licensed photos on multiple sites, including Flickr, Fotopedia, Google Images, Open Clip Art Library and Wikimedia Commons, to name just a few. You simply type in your query, check if you want to use it for commercial purposes and/or modify and adapt the photo, and then click the site you want to search. I use Flickr for almost all of my photos.
For the most part, any images you find using Creative Commons will require attribution, which means you must provide a link back to the place you got the image.
Another great site where you can get images without even needing attribution, is Morguefile. The images here are very high quality, and you can download and use them as much as you want.
Optionally, you can buy images at places like Getty Images, where you pay a fee for using the image. For most of us, this option is out of the budget, but you can get some excellent photos here.
Finding images is not that difficult, once you know where to look, but you must be sure to abide by the rules of the site you get your images from or risk legal trouble.