UrlExtension to render Gravatars

GravatarExtension.cs

using System;
using System.Security.Cryptography;
using System.Text;
using System.Web.Mvc;

public static class GravatarExtension
{
    public static string Gravatar(this UrlHelper url, string emailAddress, int size)
    {
        var baseUrl = "http://www.gravatar.com/avatar/{0}?s={1}";
        return String.Format(baseUrl, MD5Hash(emailAddress.ToLower()), size);
    }

    private static string MD5Hash(string input)
    {
        StringBuilder hash = new StringBuilder();
        MD5CryptoServiceProvider md5provider = new MD5CryptoServiceProvider();
        byte[] bytes = md5provider.ComputeHash(new UTF8Encoding().GetBytes(input));

        for (int i = 0; i < bytes.Length; i++)
        {
            hash.Append(bytes[i].ToString("x2"));
        }
        return hash.ToString();
    }
}

To implement this from a view:

<img alt="Gravatar" src="<%= Url.Gravatar(EmailAddress,40) %>" />