Looks very promising. I am going to try to follow this.
I see the developer is very open to feedback. Props to you! If i can hop into this, please change the password recommendation in the sign up form. There is really not much benefit in making complex passwords, rather tell your users to make long passwords, preferably phrases consisting of randomly picked words, or use completly auto generated passwords from a password manager. That would be good advice. PG has this in more words on the website.
Sure, if you have any questions about crypto, I am here to answer them.
As for usage. The way it’s meant to be used is for the user to turn on Auto Import and have all the photos and videos secure and end-to-end encrypted, not just select ones. Unlike competitors Stingle Photos protects your data locally too, for example from device loss or use of forensic tools to extract data. App itself doesn’t have keys to decrypt its own files when it locked.
Also if you are a high value target you can delete your encrypted keys from the server (nobody else offers such feature), but you will have to enter username, password and backup phrase on each new login on a new device.