Make sure you turn on the Beta Features preference to see this demo of Media Viewer.
Supported File Formats
Sunrise over fishing boats in Kerala.jpg
Aerial view of San Francisco
Tropical Fish Aquarium - California Academy of Sciences
This PNG file format is supported and should appear in the Media Viewer just like the other JPGs on this page.
Different aspect ratios
This Webm file format is not yet supported and should not appear in the Media Viewer yet.