First, people seek authenticity. In written content, people want to know there is a real human author thinking about every word, and in photos, people do not want to be deceived by deepfakes. It's much harder to tell in a world of AI slop, hence the rise of apps like Pangram. This is a significant negative externality.
Second, people who invest their time learning creative fields seek recognition for all the hard work they have put in. Now their skills are devalued, again because of slop. This is another negative externality.
Having more pictures and videos is nice in some ways but your head is in the sand if you think it's all roses.
Even if we're talking about creative arts, if people cannot tell the difference between AI-generated content and human-generated content, then how are they being harmed by the AI-generated creative content?
Assuming they value them equally and they prefer to have more content, AI is strictly a positive for people.
"Strictly a positive" is also a strange choice of words when the technology is ripping out the livelihoods of so many creators. I would accept "it has tradeoffs, but is a net positive over the long run". But the former is objectively false.
Automation will of course harm whichever sector of the economy remains on the manual processes, but it leads to more abundance overall.
Even the people who lose jobs due to automation are better off overall. If you personally consume goods and services produced by 100 different classes of professionals, and you yourself work in one of those 100 classes of professional work, and your job gets automated, you lose out in that narrow sense, but you gain from having the products and services produced by the other 99 classes of professionals, become more abundant through automation.
With respect to authenticity, that's easy to prove. Take coffee for example. People are willing to pay more for coffee that was made by hand. So what do companies do? They simply have the baristas make the coffee in front of the customers. Market incentives will produce efficient ways of proving authenticity.