The limits of free images
There are lots of free images available on web sites like Wikimedia Commons. I've always avoided using any of these images that aren't in the public domain, and a presentation that I just sat through made it very clear to me that I've made the right choice.
This particular presentation used lots of images that were free but required you to acknowledge the copyright holder. This doesn't sound like much of a burden, but these acknowledgments definitely distracted from the presentation that they were used in, and in a big way. Right now, I can't actually tell you anything about the content of the presentation aside from the many acknowledgements of copyright holders. And when I mentioned this to others who had sat through the same talk, they agreed that they had been similarly distracted.
So although it's perfectly reasonable for copyright holders to license content they create under any conditions that they might want to use, I'd recommend avoiding the trap of too many acknowledgements of free content distracting from other content. Or at least realize that it might be an issue for some people.