When you're in an archive, tap on any thumbnail in the grid to open a preview of that asset.
You can now flick through previews by swiping sideways. To get back to the grid, touch the back button with the archive name in the top left corner.
Simply pinch the preview to zoom in or out. When zooming in beyond 100%, the zoom will "snap" to 100% max zoom. Simply pinch again to go past that point. You can then pan around the picture using a single finger swipe.
Note: When watermarks are enabled for the archive, they will disappear the second you start zooming, since then the high-resolution image is downloaded from the FotoWeb server.
Touch the preview once to bring up the top and bottom overlay. The bottom one contains basic metadata and allows you to rate the asset. By swiping the bottom overlay up you get a bigger metadata panel with more content, including the possibility to set the asset's status and rating.