hm, i don't think 'stock' means public domain at all. there are companies that specialize in photo and film footage for movies. it's usually pretty generic images used to replace going on location and/or using an expensive model or scene. for example, you can shoot your movie on the back lot (if those still existed) in california, and use stock footage of times square in new york to establish the story location.
i was looking at some 'stock' pics the other day, and the prices ranged from $30-$99 for most of them, depending on the size and quality. off the net, they're usually watermarked and have some kind of anti-theft deal where you can't copy them to another file.