var emptyImages = doc.DocumentNode
.Descendants("img")
.Where(x => x.Attributes["src"] == null || x.Attributes["src"].Value == String.Empty)
.Select(x => x.XPath)
.ToList();
emptyImages.ForEach(xpath => {
var node = doc.DocumentNode.SelectSingleNode(xpath);
if (node != null) { node.Remove(); }
});