Access to photos

[ThomasPGH]

1) that live on Google Photos from Jellyfin

I don't think so, unless you have a permanent local copy.

2) From a local copy (accessible via the LAN) of selected photos / videos (no idea how I'd sync that to the LAN) but that's probably another discussion

You can select a local or NAS folder within the Jellyfin interface. Jellyfin of course needs access to that folder.