Don't use Google. Most images that pop up here somebody else's and you do not always have permission to use them. You could land yourself in hot water using images just found from the internet.
Try finding some FREE stock photo sites. These photos you can use without worry or legal ramifications. Please note with a lot of images you will still need to state the photographers name as a source.
Images is always a tricky issue. It's better to be safe then sorry.
Things like Flickr are a great place to find images; once again check their Creative Commons status, and make sure that you can use the images; and weather you need to add a reference back to the photographer.
Clip Art photos are usually free to use without stating a name; but always better to check.