Determine whether email has gravatar or not by checking the length of the downloaded image (6951 equals the default image).

gistfile1.cs

private static string GetGravatarUrl(
    string email,
    int size = 200,
    string defaultPicture = null)
{
    if (string.IsNullOrWhiteSpace(email))
        return null;
    byte[] b1 = Encoding.UTF8.GetBytes(email.Trim().ToLower());
    var provider = new MD5CryptoServiceProvider();
    byte[] hash = provider.ComputeHash(b1);
    string str = string.Join("", hash.Select(x => x.ToString("x2")));
    string gravatarUrl =
        string.Format("http://www.gravatar.com/avatar/{0}?s={1}&r=g", str, size);
    if (!string.IsNullOrWhiteSpace(defaultPicture))
        return gravatarUrl + "&d=" + HttpUtility.UrlEncode(defaultPicture);
    return gravatarUrl;
}

const string sql =
    @"SELECT Email FROM Emails WHERE Size IS NULL";
using (
    var connection =
        new SqlConnection(
            "Server=.\SqlExpress;Database=test;Integrated Security=true;")
    )
{
    IEnumerable<string> emails = connection.Query<string>(sql);
    foreach (string email in emails)
    {
        string url = GetGravatarUrl(email);
        var client = new WebClient();
        byte[] picture = client.DownloadData(url);
        connection.Execute(
            "UPDATE Emails SET Size = @size WHERE Email = @email",
            new {email, size = picture.Length});
    }
}