你能帮我使用c#，写一串代码，要求是：制作一个可视化类似于word的多文档应用，并插入图片作为封面，

当然可以，以下是一段参考代码：

using System;
using System.IO;
using System.Drawing;
using System.Windows.Forms;

namespace MultiDocumentApp
{
    public partial class MainForm : Form
    {
        private int documentCount = 0;

        public MainForm()
        {
            InitializeComponent();
        }

        private void newToolStripMenuItem_Click(object sender, EventArgs e)
        {
            documentCount++;
            var newDoc = new DocumentForm($"Document {documentCount}");
            newDoc.MdiParent = this;
            newDoc.Show();
        }

        private void openToolStripMenuItem_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog openFileDialog = new OpenFileDialog())
            {
                openFileDialog.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*";
                if (openFileDialog.ShowDialog() == DialogResult.OK)
                {
                    var newDoc = new DocumentForm(openFileDialog.FileName);
                    newDoc.MdiParent = this;
                    newDoc.Show();
                }
            }
        }

        private void saveToolStripMenuItem_Click(object sender, EventArgs e)
        {
            var activeDoc = this.ActiveMdiChild as DocumentForm;
            if (activeDoc != null)
            {
                activeDoc.SaveDocument();
            }
        }

        private void saveAsToolStripMenuItem_Click(object sender, EventArgs e)
        {
            var activeDoc = this.ActiveMdiChild as DocumentForm;
            if (activeDoc != null)
            {
                using (SaveFileDialog saveFileDialog = new SaveFileDialog())
                {
                    saveFileDialog.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*";
                    if (saveFileDialog.ShowDialog() == DialogResult.OK)
                    {
                        activeDoc.SaveDocumentAs(saveFileDialog.FileName);
                    }
                }
            }
        }

        private void exitToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void MainForm_Load(object sender, EventArgs e)
        {
            var coverImage = Image.FromFile(@"C:\cover.jpg");
            this.BackgroundImage = coverImage;
        }
    }

    public partial class DocumentForm : Form
    {
        private string filename = "";
        private bool isSaved = true;

        public DocumentForm(string title)
        {
            InitializeComponent();
            this.Text = title;
        }

        public DocumentForm(string filename, string title = "") : this(title)
        {
            this.filename = filename;
            this.textBox.Text = File.ReadAllText(filename);
        }

        private void textBox_TextChanged(object sender, EventArgs e)
        {
            this.isSaved = false;
        }

        public void SaveDocument()
        {
            if (string.IsNullOrEmpty(filename))
            {
                using (SaveFileDialog saveFileDialog = new SaveFileDialog())
                {
                    saveFileDialog.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*";
                    if (saveFileDialog.ShowDialog() == DialogResult.OK)
                    {
                        this.filename = saveFileDialog.FileName;
                    }
                }
            }

            if (!string.IsNullOrEmpty(filename))
            {
                File.WriteAllText(filename, this.textBox.Text);
                this.isSaved = true;
            }
        }

        public void SaveDocumentAs(string filename)
        {
            this.filename = filename;
            this.SaveDocument();
        }

        private void DocumentForm_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (!this.isSaved)
            {
                var result = MessageBox.Show("Do you want to save changes?", "Save Changes", MessageBoxButtons.YesNoCancel);
                if (result == DialogResult.Yes)
                {
                    this.SaveDocument();
                }
                else if (result == DialogResult.Cancel)
                {
                    e.Cancel = true;
                }
            }
        }
    }
}

这段代码实现了一个多文档应用，包含了文件的新建、打开、保存、另存为等操作。同时，主界面载入时会显示一张封面图片。你可以根据需要修改代码，比如增加插入图片的功能。