Images are cropped to 16:9 aspect ratio for a few reasons:
- Storyboard mode is geared for TV and feature film folks. The aspect ratio of most of their work, and traditional storyboards, is 16:9 (think your widescreen TV).
- Landscape oriented images allow a lot more to be displayed on an index card than a square or portrait image
- When embedding images inline for a document export (RTF or PDF), a landscape image doesn't take up as much space on the page.