I'm looking to send an image file from a RPI Zero (or RPI4) to a mobile phone. The image file will be sent after pressing a push-button.
The connection between the RPI and the phone is only Bluetooth (no internet or cellular data connection).
The image needs to appear as notification on the phone.
Does it need to have a dedicated app?
I tried a few methods but none actually worked, all method that I saw needed internet or a cellular connection.
OBEX push is a default Bluetooth profile, which is used to send files via Bluetooth. Every modern phone I'm aware of supports it, and with obex_push and obexctl (and a few other tools), there's good frontends for the BlueZ Bluetooth stack of Linux. You don't say which Linux distribution runs on your RPi, so you'll have to figure out how to install it.
The image needs to appear as notification on the phone.
That's 100% up to the phone to implement. Usually, when someone offers to send a file, you get a notification on your Android or iOs phone with the option to accept that file. Once the transfer is complete, you get a notification about said completion. That notification, depending on your phone's operating system, might or might not contain a preview.
