How to retrieve Someone's Avatar/Photo with agsXmpp

Question

this is what I have so far:

void xmppConnection_OnReadXml(object sender, string xml)
    {
        if (xml.Contains(XmlTags.PhotoOpen))
        {
            int startIndex = xml.IndexOf(XmlTags.PhotoOpen) + XmlTags.PhotoOpen.Length;
            int length = xml.IndexOf(XmlTags.PhotoClose) - startIndex;
            string photoHash = xml.Substring(startIndex, length);
        }
    }

I guess I can't undo the hash, but I want to the get a person's avatar/photo. How do I achieve this?