If you both turn on end-to-end encryption in the settings under “Messages”, your messages and photos are encrypted in your browser already. tellmelo then sees who writes to whom and when, but not what.
Your PIN
Your key is protected by a six-digit PIN that tellmelo does not know either. If you forget it, you set up encryption again with your password. You can no longer read earlier encrypted messages after that.
Someone running the server could guess a PIN with a lot of effort. So do not choose one that is easy to guess.