I think for many of these studies they have to go even further because it turns out that people can remember the target images and "look" at them (in their mind

I think for many of these studies they have to go even further because it turns out that people can remember the target images and "look" at them (in their mind's eye) for longer than the allotted time, even if sandwiched between neutral frames. So the target images have to be followed by random noise images that have the same frequency statistics as the target images, which interferes with your brain's recall mechanism and prevents you from "cheating."