Double-click an image file within Explorer and it won’t open in a Photo Viewer window any more, at least not by default. Instead you’ll be switched to the full-screen Windows 8 Photos app – bad news if you thought you’d escaped such hassles by using the desktop.
If you’d like to fix this, go to Control Panel > Programs > Default Programs and select Set your default programs.
Scroll down and click Windows Photo Viewer in the Programs list.
Finally, click ‘Set this program as default’ if you’d like the Viewer to open all the file types it can handle, or select the ‘Choose default’ options if you prefer to specify which file types it should open. Click OK when you’re done