Изменено на странице примеров:
HtmlDocument doc = new HtmlDocument();
doc.Load("file.htm"); //or whatever HTML file you have
HtmlNodeCollection imgs = doc.DocumentNode.SelectNodes("//img[@src]");
if (imgs == null)
return;
foreach (HtmlNode img in imgs)
{
if (img.Attributes["src"] == null)
continue;
HtmlAttribute src = img.Attributes["src"];
//Do something with src.Value
}