When a guest buys photos, do they need a payment account or special app?

No — the purchase happens right in the web gallery during checkout, before the download unlocks. Guests don't need any payment app or third-party account beyond whatever they'd use to pay on a normal website. The whole flow stays inside the album experience, which keeps the conversion path short: they've already found their photos, they're already in love with them, and the buy button is right there. Every extra step you add at that moment costs you sales, so keeping it inline is a real advantage.