GravatarHelper - inspired from http://www.danesparza.net/2010/10/using-gravatar-images-with-c-asp-net/

GravatarHelper.cs

using System.Security.Cryptography;
using System.Text;

namespace My.Library
{
    public class GravatarHelper
    {
        public static string GetGravatarImage(string email)
        {
            string hash = HashEmailForGravatar(email);
            return string.Format("https://www.gravatar.com/avatar/{0}", hash);
        }

        public static string GetGravatarImage(string email, int size)
        {
            return string.Format("{0}?size={1}", GetGravatarImage(email), size);
        }

        ///
        /// Hashes an email with MD5.  Suitable for use with Gravatar profile
        /// image urls
        ///

        /// The email address to hash
        ///
        public static string HashEmailForGravatar(string email)
        {
            // Create a new instance of the MD5CryptoServiceProvider object.
            MD5 md5Hasher = MD5.Create();

            // Convert the input string to a byte array and compute the hash.
            byte[] data = md5Hasher.ComputeHash(Encoding.Default.GetBytes(email));

            // Create a new Stringbuilder to collect the bytes
            // and create a string.
            StringBuilder sBuilder = new StringBuilder();

            // Loop through each byte of the hashed data
            // and format each one as a hexadecimal string.
            for (int i = 0; i < data.Length; i++)
            {
                sBuilder.Append(data[i].ToString("x2"));
            }

            return sBuilder.ToString();  // Return the hexadecimal string.
        }
    }
}