SHDocVwSample10

test.cs

public void ClickImage()
{
    var IE = new SHDocVw.InternetExplorer();
    IE.Visible = true;
    object URL = "http://finance.yahoo.co.jp/";
    IE.Navigate2(ref URL);
    IE.Wait();
    var doc = IE.Document as mshtml.IHTMLDocument3;
    foreach (mshtml.HTMLImg img in doc.getElementsByTagName("img"))
    {
        if (img.alt == "日経平均株価") { img.click(); } //altの値を見て選択
    };
}