Object reference not set to an instance of an object.

string htmlText = Server.UrlDecode(content);

Document document = new Document();

StyleSheet styles = new iTextSharp.text.html.simpleparser.StyleSheet();
styles.LoadTagStyle("box_wrap", "border", "1px solid #333");
styles.LoadTagStyle("box_wrap", "line-height", "22px");
styles.LoadTagStyle("box_wrap", "font-size", "12px");
styles.LoadTagStyle("li", "list-style", "none");

PdfWriter.GetInstance(document, new FileStream(strSavePath, FileMode.Create));

document.Open();
StringReader sr = new StringReader(htmlText);
List<IElement> htmlarraylist = HTMLWorker.ParseToList(new StringReader(htmlText), styles);

<img id="img1" runat="server" alt="" src="../Images/hello.png" />

<img id="img1" runat="server" alt="" src="http://localhost:1605/Images/hello.png" />