I want to get the card for seniors and it's not clear to me if it needs a photo which means wait until I arrive or if I can somehow purchase it online?
I would email OeBB.
You do not need a photo for the Vorteilscard, but you must carry a photo ID with you, matching the name for which the card was issued.
It is available either online or a via a smartphone app.
Here you will find all the details:
https://www.oebb.at/en/tickets-kundenkarten/kundenkarten/vorteilscard