You can try below code, we used it for watermark image.
System.Drawing.Image bitmap = (System.Drawing.Image)Bitmap.FromFile(Server.MapPath("image\\img_tripod.jpg")); // set image
Font font = new Font("Arial", 20, FontStyle.Italic, GraphicsUnit.Pixel);
Color color = Color.FromArgb(255, 255, 0, 0);
Point atpoint = new Point(bitmap.Width / 2, bitmap.Height / 2);
SolidBrush brush = new SolidBrush(color);
Graphics graphics = Graphics.FromImage(bitmap);
StringFormat sf = new StringFormat();
sf.Alignment = StringAlignment.Center;
sf.LineAlignment = StringAlignment.Center;
graphics.DrawString(watermarkText, font, brush, atpoint, sf);
graphics.Dispose();
MemoryStream m = new MemoryStream();
bitmap.Save(m, System.Drawing.Imaging.ImageFormat.Jpeg);
m.WriteTo(Response.OutputStream);
m.Dispose();
base.Dispose();