Sample showing how to use Spannables to replace text with Images in Mono for Android.

Activity1.cs

using Android.App;
using Android.Text;
using Android.Text.Style;
using Android.Widget;
using Android.OS;

namespace MonoDroid.TextViewWithImages
{
    [Activity(Label = "Text with Images", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        private EditText _editText;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            var textView = FindViewById<TextView>(Resource.Id.textView1);

            var imageSpan = new ImageSpan(this, Resource.Drawable.Icon); //Find your drawable.
            var spannableString = new SpannableString(textView.Text); //Set text of SpannableString from TextView
            spannableString.SetSpan(imageSpan, textView.Text.Length -1, textView.Text.Length, 0); //Add image at end of string

            textView.TextFormatted = spannableString; //Assign string to TextView (Use TextFormatted for Spannables)

            _editText = FindViewById<EditText>(Resource.Id.editText1);
            _editText.AfterTextChanged += (sender, args) => SpannableTools.AddSmiles(this, args.Editable);
        }
    }
}

Main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:minWidth="25px"
    android:minHeight="25px">
    <TextView
        android:text="Hey there, I am a TextView with an image inside!"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/textView1" />
    <EditText
        android:inputType="textMultiLine"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editText1"
        android:hint="Type some text here" />
</LinearLayout>

SpannableTools.cs

using System.Collections.Generic;
using Android.Content;
using Android.Text;
using Android.Text.Style;
using Java.Lang;

namespace MonoDroid.TextViewWithImages
{
    public static class SpannableTools
    {
        private static readonly Dictionary<string, int> Emoticons;

        static SpannableTools()
        {
            Emoticons = new Dictionary<string, int>
                             {
                                 {":-)", Resource.Drawable.Icon},
                                 {":)", Resource.Drawable.Icon}
                             };
        }

        public static bool AddSmiles(Context context, ISpannable spannable)
        {
            var hasChanges = false;
            foreach (var entry in Emoticons)
            {
                var smiley = entry.Key;
                var smileyImage = entry.Value;
                var indices = spannable.ToString().IndexesOf(smiley);
                foreach (var index in indices)
                {
                    var set = true;
                    foreach (ImageSpan span in spannable.GetSpans(index, index + smiley.Length, Java.Lang.Class.FromType(typeof(ImageSpan))))
                    {
                        if (spannable.GetSpanStart(span) >= index && spannable.GetSpanEnd(span) <= index + smiley.Length)
                            spannable.RemoveSpan(span);
                        else
                        {
                            set = false;
                            break;
                        }
                    }
                    if (set)
                    {
                        hasChanges = true;
                        spannable.SetSpan(new ImageSpan(context, smileyImage), index, index + smiley.Length, SpanTypes.ExclusiveExclusive );
                    }
                }
            }
            return hasChanges;
        }

        public static ISpannable GetSmiledText(Context context, ICharSequence text)
        {
            var spannable = SpannableFactory.Instance.NewSpannable(text);
            AddSmiles(context, spannable);
            return spannable;
        }


        public static void AddSmiley(string textSmiley, int smileyResource)
        {
            Emoticons.Add(textSmiley, smileyResource);
        }
    }

    //Taken from http://stackoverflow.com/a/767788/368379
    public static class StringExtensions
    {
        public static IEnumerable<int> IndexesOf(this string haystack, string needle)
        {
            var lastIndex = 0;
            while (true)
            {
                var index = haystack.IndexOf(needle, lastIndex);
                if (index == -1)
                {
                    yield break;
                }
                yield return index;
                lastIndex = index + needle.Length;
            }
        }
    }
}