To associate an image file with Photo Viewer, right-click any type of image file—for example, a .png, .jpg, .gif, or .bmp file—and select Open With > Choose Another App.
In the “How do you want to open this file?” window, select Windows Photo Viewer. If you don’t see it at first, scroll to the bottom of what’s listed and click the “More apps” option—it’ll show up. Select the “Always use this app to open .___ files” option, and then click “OK.”
Update: If the “Choose Another App” option doesn’t work for some reason, here’s another way you can do it: First, right-click the type of image file you want to change (for example, a PNG, JPEG, GIF, or BMP file) and select “Properties” to open the Properties window. On the General pane, click the “Change” button to the right of “Opens With” and select Windows Photo Viewer.
Windows Photo Viewer will now be the default image viewer for that type of image file. You’ll need to repeat this process for each type of image file you want to use it with. In other words, whenever you open an image that opens in the Photos app, just close the Photos app and use the “Open with” menu to associate that file type with Windows Photo Viewer. You’ll only have to do this the first time you open each new type of image file.