In Windows 10, if you have set Windows Photo Viewer as the default for all programs and the images will still open in the Photos (metro) app, this will fix that association problem.

The reason being that even if you set all defaults for Windows Photo Viewer, what is happening in the background is Windows is only set it up as the default app for TIFF files

Solution:

Run the attached reg file to merge it with your registry. You can double click it OR right-click and select "MERGE."

To verify the entries it adds, just right-click on it and open it with notepad, all it does is add associations to Windows Photo Viewer in the registry as it's broken in Windows 10

Now go to Default apps again and set Windows Photo Viewer as the default app.

Even though it still shows that it is the default for TIFF files only, any other picture like JPG, PNG, BMP, etc. will now open with the Windows Photo Viewer.

Thanks to Matrix Leader, a huge supporter of MajorGeeks!