Right you are! The line of research that sparked my guess ("big, tall trees... can have an alienating effect") has definitely not reached a settled conclusion, and the study that I linked to was a really poor choice to illustrate the issue. (I hadn't, for example, noticed that every single respondent was a resident of Brazil, renowned for its ancient rainforests!)
As Aimless points out, studying these feelings using photos is a poor proxy for the true feelings of humans.
(An aside: perhaps an immersive "virtual reality" experience would be more revealing?")
(Another aside: I think if it I was personally in such a study, images of sage steppe of Eastern Oregon might have the most positive associations for me. I've never lived there, and, while I visit a time or two per year, I spend more time recreating in the dense forests of the Cascades. But since I've had such great, liberating travel experiences in "the desert," I have extremely strong positive emotional reactions to images of it. I would probably also never choose to live there, even if my heart feels full when I visit on vacation. I have to wonder if the good memories of personal recreational experiences would trump evolutionary preferences, if any).