I've only had a quick scan through this but it seems like they were simply asking people if they were depressed. Is that correct? If so it seems very broad and unreliable. Why wouldn't they just use the standard PHQ-9 questionnaire to diagnose?
Standardized assessment has its issues (it doesn't replace a professional), but at least it works with a single definition that doesn't rely on calibrating each participant's understanding of depression. People struggling with depression often fail to recognize or be willing to admit it, and the opposite can be true as well.
By testing against participant answers, this study is actually determining if your photos correlate with saying you're depressed, which is a different and less predictable thing.