Windows 10 uses new Photos app , that is why Microsoft has hidden Windows Photo Viewer on Windows 10 fresh install.
If you upgrade to windows 10 from previous version of windows(7 or 8.1) you will have Windows Photo Viewer. Otherwise you should enable that from registry . you can reach registry – reg file from this link and unzip and run , it will be added to your registry and the end you can set windows photo viewer as default app from Setting->systems->Default apps.
