My app does not use Firebase but an external API either for authentication (custom authentication) as well as for application data.
After logging in I map user data to a FF defined user structure as suggested by the documentation.
I created a page where I open a bottom sheet to upload and change new profile picture using a sample component found within FlutterFlow (see image).
Everything works very nice as I am able to upload the picture locally, then sending it to server through a custom api call and showing the new uploaded picture in the bottom sheet.