You bring up a totally fair point for the set of images that are on it right now. But, as more users upload images that then must be tagged, the hope would be

You bring up a totally fair point for the set of images that are on it right now. But, as more users upload images that then must be tagged, the hope would be that eventually there is enough diversity in the dataset that being able to consistently recognize what's in the different images would require something approaching human intelligence.