Long time ago there was Pixave. Sadly it isn’t being developed anymore. Now there is Eagle, Inboard or References.design. Great apps, but none of them have iPad version. I am looking for something which uses iCloud for sync. And something that wouldn’t try to keep all its data synced locally, but would be smart enough to only download full images when I want to see them. Just like photos app. I am paying for Lightroom now, but boy. That’s expensive.