It doesn't quite work that way. Photos and Gallery both display any image found by the system's Media Scanner in the local directories (unless there is a ".nomedia" file in the folder). Photos may show more, since it also syncs with your Google+ Photos in the cloud, and shows those pics--but they aren't saved locally, unless you specifically download them. Gallery may or may not sync with your Google+ (aka Picasa) cloud photos--it depends on the device.
A file manager won't work because the photos aren't actually stored in the Photos or Gallery app.