Hi
If it is the Photos app that is the default viewer on windows 10 which you are refering to and you prefered the Windows Photo Viewer app of previous windows operating systems then you could do this.
Copy the following code into notepad and save it as a .reg file then merge it.
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll]
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell]
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\open]
"MuiVerb"="@photoviewer.dll,-3043"
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\open\command]
@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00,25,\
00,5c,00,53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,72,00,75,00,\
6e,00,64,00,6c,00,6c,00,33,00,32,00,2e,00,65,00,78,00,65,00,20,00,22,00,25,\
00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,00,65,00,73,00,\
25,00,5c,00,57,00,69,00,6e,00,64,00,6f,00,77,00,73,00,20,00,50,00,68,00,6f,\
00,74,00,6f,00,20,00,56,00,69,00,65,00,77,00,65,00,72,00,5c,00,50,00,68,00,\
6f,00,74,00,6f,00,56,00,69,00,65,00,77,00,65,00,72,00,2e,00,64,00,6c,00,6c,\
00,22,00,2c,00,20,00,49,00,6d,00,61,00,67,00,65,00,56,00,69,00,65,00,77,00,\
5f,00,46,00,75,00,6c,00,6c,00,73,00,63,00,72,00,65,00,65,00,6e,00,20,00,25,\
00,31,00,00,00
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\open\DropTarget]
"Clsid"="{FFE2A43C-56B9-4bf5-9A79-CC6D4285608A}"
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\print]
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\print\command]
@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00,25,\
00,5c,00,53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,72,00,75,00,\
6e,00,64,00,6c,00,6c,00,33,00,32,00,2e,00,65,00,78,00,65,00,20,00,22,00,25,\
00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,00,6c,00,65,00,73,00,\
25,00,5c,00,57,00,69,00,6e,00,64,00,6f,00,77,00,73,00,20,00,50,00,68,00,6f,\
00,74,00,6f,00,20,00,56,00,69,00,65,00,77,00,65,00,72,00,5c,00,50,00,68,00,\
6f,00,74,00,6f,00,56,00,69,00,65,00,77,00,65,00,72,00,2e,00,64,00,6c,00,6c,\
00,22,00,2c,00,20,00,49,00,6d,00,61,00,67,00,65,00,56,00,69,00,65,00,77,00,\
5f,00,46,00,75,00,6c,00,6c,00,73,00,63,00,72,00,65,00,65,00,6e,00,20,00,25,\
00,31,00,00,00
[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\print\DropTarget]
"Clsid"="{60fd46de-f830-4894-a628-6fa81bc0190d}"
This will then give you the option to use Windows Photo Viewer when you select "open with"