Windows Photo Viewer is one of the things that gets removed, or hidden from access after upgrading to Windows 10. The new Photos app takes charge as the default handler for image files in Windows 10.
The good news is Windows Photo Viewer application still exists in the same location in Windows 10, but what is removed is only its registration. Windows Photo Viewer is a light-weight app similar to the Office Picture Manager and Windows Live Photo Gallery, both of which are discontinued by Microsoft. But, Windows Photo Viewer remains an integral part of Windows.