use gravatar.com

GetAvatar.cs

public static class SignatureExtensions
{
    static Dictionary<string, string> avatars = new Dictionary<string, string>();

    public static string GetAvatar(this String email, int size = 75)
    {
        string key = email + "_" + size;
        if (!avatars.ContainsKey(key))
        {
            string avatar = "//www.gravatar.com/avatar/";
            MD5 md5Hasher = MD5.Create();
            byte[] data = md5Hasher.ComputeHash(Encoding.Default.GetBytes(email));
            StringBuilder builder = new StringBuilder();
            for (int i = 0; i < data.Length; i++)
            {
                builder.Append(data[i].ToString("x2"));
            }
            avatar += builder.ToString() + "?s=" + size;
            avatars[key] = avatar;
        }
        return avatars[key];
    }
}