Copyright is such a finicky issue. I shoot product photography for a local company where I live and it is understood that while I am the photographer, they own copyrights with the photos and I cannot resell the photos. NOW... It is also understood if i have a photo I shoot that I like I can put it in my portfolio, put in on my website, promotionals, etc... The advertising manager does the same thing with her personal portfolio where when she uses my photographs in her layouts and catalogs, etc... For her it's a reflection of her layout/graphic design skills and for me, it's a reflection of my photography skills... We just cant profit off the photos any more than whats in our paycheck.
With just about all my other jobs, I have written into my contracts I own copyrights... On weddings, portraits, architecture, etc... Now also the rough rule of thumb of "if you alter the photo 5% then the photo is yours" is also sketchy but used in everyday life, SO, if you shoot a photo for a client, they like it, buy it, and then you turn it into fine art, clip things out, do post production to the photo, etc... Then as long as you keep your nose clean and it's not perfectly recognizable, then it is what it is.