Well, I still think the whole premise in the @steemcleaners comment is wrong:

By doing so you are claiming to be the content creator.

When a photo is licenced in a way which neither requires payment nor attribution, it shouldn't be expected by the post creator to still have to add that unnecessary attribution to the post. It's distracting and may hurt the visual appeal of the post.

By not including attribution or source one is simply claiming that the picture is either one's own or legal to use without attribution.