You can directly grab the attribute if you use the HtmlNavigator
instead.
//Load document from some html string
HtmlDocument hdoc = new HtmlDocument();
hdoc.LoadHtml(htmlContent);
//Load navigator for current document
HtmlNodeNavigator navigator = (HtmlNodeNavigator)hdoc.CreateNavigator();
//Get value from given xpath
string xpath = "//div[@id='topslot']/a/img/@src";
string val = navigator.SelectSingleNode(xpath).Value;