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On your iPhone only. There is no account and no cloud, so nothing syncs between devices and nobody else can see your keys. See the privacy policy for the detail.
Five keys, with everything working: names, notes, photos and search. Unlimited Keys lifts the limit for good with a single payment. There is no subscription and nothing renews. Keys you saved before buying stay exactly where they are whether you buy or not.
Open the menu in the top right of the drawer, choose Unlimited Keys, then Restore purchase. The unlock belongs to your Apple Account, so make sure you are signed in with the one that bought it.
The current version picks from your photo library. Take the photo with the Camera app first, then attach it from the picker. Camera capture inside the app is on the list for a later version.
If the photo lives in iCloud and the device is offline, the download can fail — the app will say so rather than attach nothing. Check your connection and pick it again. Some unusual formats cannot be shown; a photo taken with the iPhone camera always works.
Swipe the key's row to the left, or open the key and use Delete key at the bottom of the form. Deleting removes the entry and its photo. The physical key is on its own.
Search matches the key's name, its note, and what it opens — so “padlock”, “shed” or “green fob” all work if that text is on the key. Clearing the search field always brings the whole drawer back.