The researchers compiled a database of around 10,000 images, and then had hundreds of test subjects sit through a subset of the photos, some of which were repeated. They were told to flag the ones they had seen before, essentially testing how well they remembered each image. The ones that were most memorable? Those that had humans in them, followed by photos taken on an personal scale (interiors and small objects), with large landscapes coming in last. Across all of their testers, this pattern staid true.