I check your code there is an issue with Image

You mention width is 1; also, the image is not fitted so there's some spacing

So i have added your code on my system with different imageand its working fine

I have attached screenshot Please check

enter image description here

import 'package:flutter/material.dart';

class CardScreen extends StatefulWidget {
  const CardScreen({super.key});

  @override
  State<CardScreen> createState() => _CardScreenState();
}

class _CardScreenState extends State<CardScreen> {
  String selectedCurrency = 'USD';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: Column(
        mainAxisAlignment: MainAxisAlignment.start,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: [
          Stack(
            children: [
              Image.network(
                "https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQ8lF2jbNFBy7X4D6F43tRiCxG2oRWLP9v8LQ&s",
                height: 360,
                width: MediaQuery.of(context).size.width,
                fit: BoxFit.cover,
              ),
              SafeArea(
                child: Padding(
                  padding: EdgeInsets.symmetric(horizontal: 50),
                  child: Column(
                    children: [
                      SizedBox(height: 40),
                      Text(
                        'Your card balance',
                      ),
                      SizedBox(
                        height: 15,
                      ),
                    ],
                  ),
                ),
              )
            ],
          ),
          Text('My Card'),
        ],
      ),
    );
  }
}