You were already aware of the fact that you couldn’t just use any photo, you found on the Internet. Good 🙂
I have a favourite place, where I look for photos: http://www.compfight.com/
Just put a checkmark in Commercial, and you are good. Be sure to mention who took the photo and link to their site at Flickr.
There are other pages, you can find free pictures. Look for both Free and Royalty free photos. The latter does not necessarily mean that they are free, but once you’ve bought the photo, it can be used freely.
I almost always find what I’m searching for at CompFight.