And here is how the bad information starts, viewed in real time. Matt just did a video answering the question, does the use of stock images negatively affect a sites ranking. Reporting of it here. [searchengineland.com] In the video Matt answers that no, stock images don't influence the rankings either way. Then Matt compliments the person who asked the question by telling him that it's an interesting metric to possibly look at. That was it. Matt complimented the guy for asking a good question. Then some blogger guy starts up the FUD machine (because FUD is good for blog traffic) and tweets that overusing stock images could be a negative quality signal in the future.
And that my friends is how crap information is created and becomes yet another SEO myth. Matt compliments a guy for asking a good question. The SEO bloggers start beating the drums that stock images could be a negative signal in the future.