Just found this old post with no answer. The solution is quite simple, assuming, I did understand the initial posting…
So I repeat: A random Grandpa has shot some photos which he wants to view on the TV, which is connected to a RaspberryPi with installed Kodi via OpenElec. The Grandpa stores his photos, taken by an ordinary camera on a usual Computer (MacOS/WinPC/Ubuntu/…) which is not connected to the Raspberry Pi or the TV, but has connection via LAN/WLAN to the Raspberry Pi.
From the Kodi side, the grandpa just needs to go in the “pictures” Menu and add a new picture source. This is done via “Add Pictures…” and browse to the path, the photos/images are stored.
So, this path could be a path on a NAS, or a path locally on the RaspberryPI which is set up to sync with the syncthing-service, running on the RaspberryPI.
Actually, thats all, beside the grandpa’s computer technican needs to install and set up syncthing on the RaspPI and exchange the folder-IDs for the Computer (MacOS/WinPC/Ubuntu/…) where the other instance of syncthing is running, checking the folder where grandpa stores his images from the camera.
This way all the files/images grandpa saves from the camera to his computer will automaticly synced into the syncthing-folder on the RaspBerry PI, which is set up as a Picture Source inside Kodi.
Now its possible to view (or whatever is possible with Kodi) these images on the TV screen.
Hope, this non-technically explanaition helps realising the initial idea.