Mobile phones are becoming platforms for personal sensing, computing and communication. This paper proposes TagSense, a mobile phone system that uses sensing to automatically tag images. TagSense senses people, activities and contexts in pictures and merges them to create tags on-the-fly. It was tested on 8 Android phones with 200 pictures, showing it can effectively tag images. Compared to Apple and Google tagging systems, TagSense provides valuable tagging by using additional sensing dimensions, especially as devices and sensing algorithms improve.