Hey, you are right about that. In the end it is up to you. Steemit does not force anyone to do so, although steemcleanaers guide specifically says:

"steemcleaners would like to see all images that are not original cited. Even if it is just with Public Domain, CC0 or a link to where you found the image.
There are literally millions of high-resolution photo’s available on sites like pixabay.com that are CC0 or public domain."