Handle different response type for the same route -Flutter

I have an API which send a random photo each time you call it. But if you've seen all the photos recently, it responds a json object saying that no more photo is available. In my Flutter App, fetching the photo everytime the button is pressed works fine (thx StackOverflow) but i honestly don't see how to handle when the response is a json, because my flutter app can't know when the response will be json, so I can't use a model. Here is the working code if a photo is available :

import 'package:flutter/material.dart';
import 'dart:math';

class ImageDialog extends StatelessWidget {
  @override
  @override
  Widget build(BuildContext context) {
    return Dialog(
        child: FutureBuilder(
      future: _loadImage(),
      builder: (BuildContext context, AsyncSnapshot<Image> image) {
        if (image.hasData) {
          return image.data; // image is ready
        } else if (image.hasError) {
          return Text("No photo available"); // placeholder
        } else {
          return Text("Impossible");
        }
      },
    ));
  }

  Future<Image> _loadImage() async {
    
      return Image.network(
        'http://192.168.1.19:5000/photo',
        headers: {
          'Content-Type': 'application/json',
          'Accept': 'application/json',
          'Authorization':
              'Bearer token',
        },
        key: ValueKey(new Random().nextInt(100)),
      );
    
  }
}

Thx in advance :)