Robocopy is great if you have a "master / slave" setup - one side must always mirror the other side exactly. SyncToy can do bi-directional sync (no master), which is what I thought of when I read "replicator".
You want to process "new photos" ... if I was doing something like this, I might do it in a "batch" fashion, moving files around according to their status. New photos land in one directory and get processed e.g. hourly. Use ImageMagick to process all the new files, with the copies landing in the network drive. I'd move the new originals to an archive after processing, perhaps by name. That way, you can just run ImageMagick on all the files in the directory, rather than trying to detect the new files in a crowded directory.