I did an advertisement for a business a couple of years ago using an image we found on Flickr. The photography was great, the subject matter was perfect for our theme (relating to a sci-fi collectibles business), and the ad was successful.
Our approach was a little different, however.
First, we contacted the owner of the photo to get permission. Our attitude going in was that we were planning to use this for commercial purposes, so we offered to pay the person a competitive rate for their work (much to their delight). We negotiated payment and put the photographer's name in the ad.
In the end, we ended up with a loyal customer who brought more friends to the business, and a fun, memorable ad.
So, sure. I'm in favor of people sharing their photos. And if you enjoy someone's work enough to want to use it commercially, why not get permission and offer reasonable compensation? There's a chance you'll build a good relationship, and you'll encourage them to do more work.
Honestly, it seems like it would be more costly to rip them off. The initial work might be free, but the potential lost revenue from angry customers and would-be customers would probably more than offset any savings.