If Windows Photo Gallery does not have an option to set the default save path then you can create a directory symbolic link or junction point instead:
Close Photo Gallery
Delete
C:\Users\<Username>\AppData\Local\Microsoft\Windows Photo Gallery\Original Images
At an admin (elevated) command prompt run:
mklink /d "C:\Users\<Username>\AppData\Local\Microsoft\Windows Photo Gallery\Original Images" "D:\Original Images"
or if that doesn't work then:
mklink /j "C:\Users\<Username>\AppData\Local\Microsoft\Windows Photo Gallery\Original Images" "D:\Original Images"
(Obviously edit all paths as required.)