using System.Windows.Media;
using System.Windows.Media.Imaging;

        /// <summary>
        /// <para>Ecrit un texte sur une <see cref="BitmapSource"/> et retourne la nouvelle image rendue.</para>
        /// </summary>
        /// <param name="image">Image source</param>
        /// <param name="text">Texte à rendre</param>
        /// <param name="location">Point d'insertion du texte</param>
        /// <param name="size">Taille de police</param>
        /// <param name="color">Couleur du texte</param>
        public static BitmapSource Write(BitmapSource image, string text, Point location,  int size, SolidColorBrush color) 
        {
           
            // Cible de rendu:
            RenderTargetBitmap renderer = new RenderTargetBitmap(
                image.PixelWidth, // Largeur
                image.PixelHeight, // Hauteur
                image.DpiX, // Résolution X
                image.DpiY, // Y
                PixelFormats.Pbgra32); // Codage pixels


            #region Traitement de l'image dans un DrawingVisual
            DrawingVisual drawingVisual = new DrawingVisual();

            // Retrieve the DrawingContext in order to create new drawing content.
            DrawingContext drawingContext = drawingVisual.RenderOpen();

            // draw the image and text in the DrawingContext.
            Rect rect = new Rect(new Size(image.Width, image.Height));
            drawingContext.DrawImage(image, rect);
            FormattedText formatted = new FormattedText(
                text,
                System.Threading.Thread.CurrentThread.CurrentUICulture,
                FlowDirection.LeftToRight,
                new Typeface("Verdana"),
                size,
                color);
            drawingContext.DrawText(formatted, location);
            // Persist the drawing content.
            drawingContext.Close(); 
            #endregion


            renderer.Render(drawingVisual);

            return renderer;
          
        }

Il s'agit d'une reprise de la source trouvée sur: http://social.msdn.microsoft.com/Forums/en/wpf/thread/e1b4568b-c250-487d-bd1e-a903775a3d90