Print It Before You Fly
Visa on arrival is a paper form at Tribhuvan.
Most travellers get their Nepali visa on arrival, and that process still runs on a paper application with a photo attached. A file on your phone does not help you at the desk.
Bring at least one printed 35x45mm photo — two is safer, since the same photo size is used if you extend the visa at an immigration office later. Kiosks and photo booths exist at the airport, but queues at arrival are exactly when you do not want to depend on them.
