This paper explores photo organization within an event photo stream, i.e. the chronological sequence of photos from a single event. The problem is important: with the advent of inexpensive, easy-to-use photo capture devices, people can take a large number of photos per event. A family trip, for example, may include hundreds of photos. In this work, we have developed a photo browser that uses automatically segmented groups of photos - -referred to as chapters - -to organize such photos. The photo browser also affords users with a drag-and-drop interface to refine the chapter groupings. We conducted an exploratory study of 23 college students with their 8096 personal photos from 92 events, to understand the role of different spatial organization strategies in our chapter-based photo browser, in performing storytelling, photo search and photo set interpretation tasks. We also report novel insights on how the subjects organized their photos into chapters. We tested three layout strategies: bi-level, grid-stacking and space-filling, against a baseline plain grid layout. We found that subjects value the chronological order of the chapters more than maximizing screen space usage and that they value chapter consistency more than the chronological order of the photos. For automatic chapter groupings, having low chapter boundary misses is more important than having low chapter boundary false alarms; the choice of chapter criteria and granularity for chapter groupings are very subjective; and subjects found that chapter-based photo organization helps in all three tasks of the user study. Users preferred the chapter-based layout strategies to the baseline at a statistically significant level, with the grid-stacking strategy preferred the most.