Within a certain threshold, simply looking at an image will register an amount of activity in anybody's brain, but I think the context of the viewing situation, combined with the power differential of the individual controlling the slide show, obviously plays more heavily on the mind than the subject matter of the image itself.
For example, a kidnapper asking for a million dollars, and showing you a picture of your own child tied to a chair, while cocking a pistol and pointing it at you, is going to stress you a lot more as a credible threat, than a grad student in a doctor's office asking you to "think about things," while showing you a picture of some random crying child, while an MRI clicks and hums around you.
But for some people (probably many people), the grad student with the clipboard and the MRI, is actually kind of scary in it's own strange way.