How to update image in gtk.Button with GTK#

Question

Well I was working on simple application for starting/stopping and restarting my LAMP in mono (just to get knowing the GUI development with mono bit more), so to have less buttons I decided to have one button for starting and stopping the servers. In the GUI designer I added a start button with icon, the problem is that updating the button label is easy, but changing the image to Stock.MediaStop is a bit of a problem. So How to change the stock image in a button on click event (it doesn't meter on what kind of event it is actually). Here some code: The GUI XML for a button:

<widget class="Gtk.Button" id="button1">
       <property name="MemberName" />
       <property name="CanFocus">True</property>
       <property name="Type">TextAndIcon</property>
       <property name="Icon">stock:gtk-media-play Menu</property>
       <property name="Label" translatable="yes">Start</property>
       <property name="UseUnderline">True</property>
       <signal name="Clicked" handler="OnMysqlServerStartStop" />
</widget>

And here how MediaDevelop made the Stock button with custom text:

// Container child hbox1.Gtk.Box+BoxChild
        this.hbuttonbox1 = new Gtk.HButtonBox();
        this.hbuttonbox1.Name = "hbuttonbox1";
        // Container child hbuttonbox1.Gtk.ButtonBox+ButtonBoxChild
        this.button1 = new Gtk.Button();
        this.button1.CanFocus = true;
        this.button1.Name = "button1";
        this.button1.UseUnderline = true;
        // Container child button1.Gtk.Container+ContainerChild
        Gtk.Alignment w2 = new Gtk.Alignment(0.5F, 0.5F, 0F, 0F);
        // Container child GtkAlignment.Gtk.Container+ContainerChild
        Gtk.HBox w3 = new Gtk.HBox();
        w3.Spacing = 2;
        // Container child GtkHBox.Gtk.Container+ContainerChild
        Gtk.Image w4 = new Gtk.Image();
        w4.Pixbuf = Stetic.IconLoader.LoadIcon(this, "gtk-media-play", Gtk.IconSize.Menu, 16);
        w3.Add(w4);
        // Container child GtkHBox.Gtk.Container+ContainerChild
        Gtk.Label w6 = new Gtk.Label();
        w6.LabelProp = Mono.Unix.Catalog.GetString("Start");
        w6.UseUnderline = true;
        w3.Add(w6);
        w2.Add(w3);
        this.button1.Add(w2);
        this.hbuttonbox1.Add(this.button1);