<script type="text/javascript">
    function newpic() {
        var bd = document.getElementById("bigimage").href = "Images/3.jpg"
        var bs = document.getElementById("smallimage").src = "Images/4.jpg"
    }
</script>

  ClientScript.RegisterStartupScript(this.GetType(), "", "<script type='text/javascript'>newpic();</script>");

------解决方案--------------------
在页面上放个控件，比如literal，把你要生成的js在服务端包装好，ru：
C# code
public static string GetJS(string id)
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("<script type=\"text/javascript\">");
            sb.Append("function OpenDialog()");
            sb.Append("{");
            sb.Append("var users=window.showModalDialog('SelectUsers.aspx','0','dialogWidth:300px;dialogHeight:450px');");
            sb.Append(string.Format("document.getElementById('{0}').value=users;",controlid));
            sb.Append("}");
            sb.Append("</script>");
            return sb.ToString(); 
        }

------解决方案--------------------
.aspx中：
  <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />

  <img id="smallimage" src="" />
  <asp:Label ID="Label1" runat="server" Text=""></asp:Label>

.cs中：

 protected void Button1_Click(object sender, EventArgs e)
   {
       StringBuilder strSb = new StringBuilder();
       string strPic= "Images/4.jpg";

       strSb.Append("<script type='text/javascript'>");
       strSb.Append("document.getElementById('smallimage').src =");
       strSb.Append(strPic);
       strSb.Append("</script>");
   }