A random website claiming "this is my stuff and under CC license" is less reliable than a written contract.

How do you prove that the license really was granted if I take the website down? Convince a court that archive.org or a printed screenshot is evidence? If I falsely claimed to own the rights to the images, and you get sued by the true owner, can you show you acted in good faith and did enough to verify the license? If you have a contract with me and I lied, you probably would have better chances of recovering your damages from me.

It would be interesting to see how many companies would pay a small fee for this if you asked.