This should work better:

foreach (HtmlNode link in doc.DocumentNode.SelectNodes("//a[@href]"))
        {
            HtmlAttribute att = link.Attributes["href"];
            att.Value = RepairHyperlinkAddress(att.Value, web);
        }

foreach (HtmlNode link in doc.DocumentNode.SelectNodes("//img[@src]"))
        {
            HtmlAttribute att = link.Attributes["src"];
            att.Value = RepairHyperlinkAddress(att.Value, web);
        }