private void btnInsertImg_Click(object sender, EventArgs e)
{
   try
   {
      if (File.Exists(txtImgFile.Text))
      {
         Image img = Image.FromFile(txtImgFile.Text);
         RTB_InsertImg.InsertImage(this.richTextBox1, img);
      }
   }
   catch (Exception ex)
   {
      MessageBox.Show("RichTextBox仅支持BMP, JPEG, GIF，ICO，PNG和TIFF格式的图像.");
   }
}

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;
using System.Drawing;
using System.Windows.Forms;
using System.Collections.Specialized;
using System.Drawing.Imaging;
using System.IO;

namespace TestProject
{
   /// <summary>
   /// RichTextBox插入图片使用的静态类
   /// 资料来源:www.baidu.com
   /// </summary>
   public class RTB_InsertImg
   {
      public const int
      EmfToWmfBitsFlagsDefault = 0x00000000,
      EmfToWmfBitsFlagsEmbedEmf = 0x00000001,
      EmfToWmfBitsFlagsIncludePlaceable = 0x00000002,
      EmfToWmfBitsFlagsNoXORClip = 0x00000004;
      
      [DllImport("gdiplus.dll")]
      public static extern uint GdipEmfToWmfBits(IntPtr _hEmf, uint _bufferSize, byte[] _buffer, int _mappingMode, int _flags);
      
      private struct RtfFontFamilyDef
      {
         public const string Unknown = @"\fnil";
         public const string Roman = @"\froman";
         public const string Swiss = @"\fswiss";
         public const string Modern = @"\fmodern";
         public const string Script = @"\fscript";
         public const string Decor = @"\fdecor";
         public const string Technical = @"\ftech";
         public const string BiDirect = @"\fbidi";
      }
      
      private const int MM_ISOTROPIC = 7;
      private const int MM_ANISOTROPIC = 8;
      private const int HMM_PER_INCH = 2540;
      private const int TWIPS_PER_INCH = 1440;
      
      private const string FF_UNKNOWN = "UNKNOWN";
      
      private const string RTF_HEADER = @"{\rtf1\ansi\ansicpg1252\deff0\deflang1033";
      private const string RTF_DOCUMENT_PRE = @"\viewkind4\uc1\pard\cf1\f0\fs20";
      private const string RTF_DOCUMENT_POST = @"\cf0\fs17}";
      private const string RTF_IMAGE_POST = @"}";
      
      private static HybridDictionary rtfFontFamily;
      
      static RTB_InsertImg()
      {
         rtfFontFamily = new HybridDictionary();
         rtfFontFamily.Add(FontFamily.GenericMonospace.Name, RtfFontFamilyDef.Modern);
         rtfFontFamily.Add(FontFamily.GenericSansSerif.Name, RtfFontFamilyDef.Swiss);
         rtfFontFamily.Add(FontFamily.GenericSerif.Name, RtfFontFamilyDef.Roman);
         rtfFontFamily.Add(FF_UNKNOWN, RtfFontFamilyDef.Unknown);
      }
      
      private static string GetFontTable(Font _font)
      {
         StringBuilder _fontTable = new StringBuilder();
         _fontTable.Append(@"{\fonttbl{\f0");
         _fontTable.Append(@"\");
         if (rtfFontFamily.Contains(_font.FontFamily.Name))
         _fontTable.Append(rtfFontFamily[_font.FontFamily.Name]);
         else
         _fontTable.Append(rtfFontFamily[FF_UNKNOWN]);
         _fontTable.Append(@"\fcharset0 ");
         _fontTable.Append(_font.Name);
         _fontTable.Append(@";}}");
         return _fontTable.ToString();
      }
      
      ///
      /// 在RichTextBox当前光标处插入一副图像。
      ///
      /// 多格式文本框控件
      /// 插入的图像
      public static void InsertImage(RichTextBox rtb, Image image)
      {
         StringBuilder _rtf = new StringBuilder();
         _rtf.Append(RTF_HEADER);
         _rtf.Append(GetFontTable(rtb.Font));
         _rtf.Append(GetImagePrefix(rtb, image));
         _rtf.Append(GetRtfImage(rtb, image));
         _rtf.Append(RTF_IMAGE_POST);
         rtb.SelectedRtf = _rtf.ToString();
      }
      
      private static string GetImagePrefix(RichTextBox rtb, Image _image)
      {
         float xDpi;
         float yDpi;
         using (Graphics _graphics = rtb.CreateGraphics())
         {
            xDpi = _graphics.DpiX;
            yDpi = _graphics.DpiY;
         }
         
         StringBuilder _rtf = new StringBuilder();
         int picw = (int)Math.Round((_image.Width / xDpi) * HMM_PER_INCH);
         int pich = (int)Math.Round((_image.Height / yDpi) * HMM_PER_INCH);
         int picwgoal = (int)Math.Round((_image.Width / xDpi) * TWIPS_PER_INCH);
         int pichgoal = (int)Math.Round((_image.Height / yDpi) * TWIPS_PER_INCH);
         _rtf.Append(@"{\pict\wmetafile8");
         _rtf.Append(@"\picw");
         _rtf.Append(picw);
         _rtf.Append(@"\pich");
         _rtf.Append(pich);
         _rtf.Append(@"\picwgoal");
         _rtf.Append(picwgoal);
         _rtf.Append(@"\pichgoal");
         _rtf.Append(pichgoal);
         _rtf.Append(" ");
         return _rtf.ToString();
      }
      
      private static string GetRtfImage(RichTextBox rtb, Image _image)
      {
         StringBuilder _rtf = null;
         MemoryStream _stream = null;
         Graphics _graphics = null;
         Metafile _metaFile = null;
         IntPtr _hdc;
         try
         {
            _rtf = new StringBuilder();
            _stream = new MemoryStream();
            using (_graphics = rtb.CreateGraphics())
            {
               _hdc = _graphics.GetHdc();
               _metaFile = new Metafile(_stream, _hdc);
               _graphics.ReleaseHdc(_hdc);
            }
            using (_graphics = Graphics.FromImage(_metaFile))
            {
               _graphics.DrawImage(_image, new Rectangle(0, 0, _image.Width, _image.Height));
            }
            IntPtr _hEmf = _metaFile.GetHenhmetafile();
            uint _bufferSize = GdipEmfToWmfBits(_hEmf, 0, null, MM_ANISOTROPIC, EmfToWmfBitsFlagsDefault);
            byte[] _buffer = new byte[_bufferSize];
            uint _convertedSize = GdipEmfToWmfBits(_hEmf, _bufferSize, _buffer, MM_ANISOTROPIC, EmfToWmfBitsFlagsDefault);
            for (int i = 0; i < _buffer.Length; ++i)
            {
               _rtf.Append(String.Format("{0:X2}", _buffer[i]));
            }
            
            return _rtf.ToString();
         }
         finally
         {
            if (_graphics != null)
            _graphics.Dispose();
            if (_metaFile != null)
            _metaFile.Dispose();
            if (_stream != null)
            _stream.Close();
         }
      }
   }
}