Hello all

I'm currently building a website for a client. It's for a commercial generator business and as the display of previous work he's done, he wants me to use logos of major corporations (i.e. Warner Brothers, Showtime, HBO, ect...). My first question is does this violate copyright if I find a logo on a Royalty Free photo site. I would think if it's on there, that means it is ok to use. But considering I can not find any of the logos I need on the usual suspects, Big Stock, iStock, fotolia and Microsoft , then I'm guessing there is a good chance they are not to open to sharing these images. I've seen some of these logos on other websites, but they were major corporations as well.
Does anyone have any knowledge or insight regarding my dilemma?
Thanks in advance and have a fantastic day!
Dawn