How to add Image Asset as a Background in Flutter?

Question

I have a vector that I want to be part of the background of my screen but I don't know how will I implement it. I tried inserting image.asset in the children but it is being placed above my elements in the screen. Pease help. This is my existing code right now:

import 'package:ehatid_driver_app/signup.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'Welcome/components/body.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({
    Key? key,
  }) : super(key: key);

  @override
  State<LoginScreen> createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {

  //text controller
  final _emailController = TextEditingController();
  final _passwordController = TextEditingController();

  Future signIn() async {
    await FirebaseAuth.instance.signInWithEmailAndPassword(
        email: _emailController.text.trim(),
        password: _passwordController.text.trim(),
    );
  }

  @override
  void dispose() {
    _emailController.dispose();
    _passwordController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xFFFFFCEA),
      body: SafeArea(
        child: Center(
          child: SingleChildScrollView(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                Image.asset("assets/images/illus14.png",
                    width: 250
                ),
                Text(
                  "Sign in to your account",
                  style: TextStyle(fontFamily: 'Montserrat', fontSize: 28, letterSpacing: -2, fontWeight: FontWeight.bold),
                ),
                SizedBox(height: 10),
                Text(
                  "Welcome Back! Ready for your next ride?", textAlign: TextAlign.center,
                  style: TextStyle(fontFamily: 'Montserrat', fontSize: 13, color: Color(0xff272727), letterSpacing: -0.5, fontWeight: FontWeight.w500),
                ),
                SizedBox(height: 30),
                Padding(
                  padding: const EdgeInsets.symmetric(horizontal: 25),
                  child: Container(
                    decoration: BoxDecoration(
                      color: Colors.white,
                      boxShadow: [
                        BoxShadow(
                            color: Color(0xffeeeeee),
                            blurRadius: 10,
                            offset: Offset(0,4)
                        ),
                      ],
                      borderRadius: BorderRadius.circular(15),
                    ),
                    child: Padding(
                      padding: const EdgeInsets.only(left: 20),
                      child: TextField(
                        controller: _emailController,
                        decoration: InputDecoration(
                          border: InputBorder.none,
                          hintText: "Email",
                        ),
                      ),
                    ),
                  ),
                ),
                SizedBox(height:10),
                Padding(
                  padding: const EdgeInsets.symmetric(horizontal: 25),
                  child: TextField(
                        controller: _passwordController,
                        obscureText: true,
                        decoration: InputDecoration(
                          enabledBorder: OutlineInputBorder(
                            borderSide: BorderSide(color: Colors.white),
                            borderRadius: BorderRadius.circular(15),
                          ),
                          focusedBorder: OutlineInputBorder(
                            borderSide: BorderSide(color: Color(0xFFFED90F),),
                            borderRadius: BorderRadius.circular(15),
                          ),
                          hintText: "Password",
                          fillColor: Colors.white,
                          filled: true,
                        ),
                      ),
                    ),
                SizedBox(height: 20),
                Padding(
                  padding: const EdgeInsets.symmetric(horizontal: 25),
                  child: GestureDetector(
                    onTap: signIn,
                    child: Container(
                      padding: EdgeInsets.all(20),
                      decoration: BoxDecoration(
                        color: Color(0xFFFED90F),
                          borderRadius: BorderRadius.circular(50),
                      ),
                      child: Center(
                        child: Text(
                          "Sign in",
                          style: TextStyle(
                              color: Colors.white, fontFamily: 'Montserrat', fontSize: 16
                          ),
                        ),
                      ),
                    ),
                  ),
                ),
                SizedBox(height: 20),
                Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text("Don't have an account yet?", style: TextStyle(
                        color: Color(0xFF494949), fontFamily: 'Montserrat', fontSize: 16, letterSpacing: -0.5, fontWeight: FontWeight.w500
                      ),
                    ),
                    TextButton(
                      onPressed: () {
                        Navigator.pushReplacement(context, MaterialPageRoute(
                          builder: (_) => SignUpPage(),
                        ),
                        );
                      },
                      child: Text("Register", style: TextStyle(fontFamily: 'Montserrat', fontSize: 16,
                        letterSpacing: -0.5, fontWeight: FontWeight.w600,
                        decoration: TextDecoration.underline, color:Color(0xFFFEDF3F),
                      ),
                      ),
                    ),
                  ],
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

This the output of the above code:

However, this is what I wanted it to look like. That yellow vector is an image, btw.