Simple C# method to create the hash of the email address and return Gravatar image URL for a supplied email address. As per official documentation - https://en.gravatar.com/site/implement/images/

Gravatar.cs

public static string GetGravatarImageUrl(string emailAddress)
{
    // Create MD5 Hash of email address
    var md5 = System.Security.Cryptography.MD5.Create();
    byte[] inputBytes = System.Text.Encoding.ASCII.GetBytes(emailAddress.Trim().ToLower());
    byte[] hash = md5.ComputeHash(inputBytes);

    // Create lower-case hex string
    var sb = new System.Text.StringBuilder();
    for (int i = 0; i < hash.Length; i++)
    {
        sb.Append(hash[i].ToString("x2"));
    }

    // Build URL
    return string.Concat("http://www.gravatar.com/avatar/", sb);
}