Open an event and click ▣ Photos. The photo viewer opens on the first picture.
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Move between photos with the arrows on the picture (or your keyboard's arrow keys).
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Click ▶ Slideshow to play them automatically (Space starts and stops it), or ⛶ for full screen (F key).
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Under the photo, the tabs show About this photo (caption and who added it), All photos (a grid of everything), Participants, and Discussion for that one photo.
Tip: Want only one person's pictures? Click the contributor's name, then "Show everyone's photos" to go back.