Save this to xyzzy.reg, then double click it.

Windows Registry Editor Version 5.00

; ================================================
; Enable Classic Windows Photo Viewer + Set Defaults
; For Windows 10 and Windows 11
; ================================================

; --- Enable the Photo Viewer application ---

[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]
@="\"%SystemRoot%\\System32\\rundll32.exe\" \"%ProgramFiles%\\Windows Photo Viewer\\PhotoViewer.dll\", ImageView_Fullscreen %1"

[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\open\DropTarget]
"Clsid"="{FFE2A43C-56B9-4bf5-9A79-CC6D4285608A}"

[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\print]
"NeverDefault"=""

[HKEY_CLASSES_ROOT\Applications\photoviewer.dll\shell\print\command]
@="\"%SystemRoot%\\System32\\rundll32.exe\" \"%ProgramFiles%\\Windows Photo Viewer\\PhotoViewer.dll\", ImageView_Fullscreen %1"

; --- Set as default for common image formats ---

[HKEY_CURRENT_USER\Software\Classes\.jpg]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.jpeg]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.jpe]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.jfif]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.png]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.bmp]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.dib]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.gif]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.tif]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.tiff]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.ico]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.wdp]
@="PhotoViewer.FileAssoc.Tiff"

[HKEY_CURRENT_USER\Software\Classes\.webp]
@="PhotoViewer.FileAssoc.Tiff"