Optimization is the practice of doing what it takes to make a website attractive and engaging to its visitors. Although you left the SE out of optimization, I suspect that's where you'd like to lead the discussion. However, this forum is about creating good content for the people that visit websites: content that makes them return to them, refer them to other people, all of whom will potentially become satisfied customers (and that goes to it whether you are selling something or not).
In that respect, Greg's "theory" is 100% correct. How many times have you purchased a product from a website without seeing a picture of what you are buying? Tutorials, portfolios, and service sites have thumbnails that explode into larger views, screenshots, diagrams, and infographics all to make their sites more engaging to their visitors.
What Greg says is that a site would certainly be more appealing to its visitors if it offered original art rather than the over-used free clipart and stock images you see on many websites. I believe he's correct in his assumption. Proof of that is shown not only by the recent popularity of infographics but also by such sites as "The Oatmeal" and "Zen Garden" where art work is what draws visitors and brings them back for a second look. Any type of originality shouldn't be overlooked in creating website content, just as most designers work to add originality to the website they create.