How do I get images to appear on buttons in Glade Designer?

The steps below should work (from the look of your question you can start at step 5 but I've included all of them for future users).

1. Make sure the icon image you want to use is in your_app/data/media
2. select the widget you want to apply the image to in glade
3. choose icon widget in edit image on the right tool button properties panel to add an image widget to your button
4. edit the image widget you just created to add an image using the right panel (select file name in edit image and browse to your image
5. prefix the file name that appears in file name with ../media/ so glade can see it.

You can see a screencast of this process from about 33 minutes onwards in the create your first app video Jono Bacon released.

I believe this is because by default (not sure if it's a GNOME-wide or GTK setting), no images are shown in buttons, so I'm not sure there is a way around it, either.

This Stackoverflow question hints at stock images not shown in buttons because of that, which I assume it applies to custom images too.

You might try the workaround mentioned in that question to see if it works for you:

from gi.repository import Gtk

settings = Gtk.Settings.get_default()
settings.props.gtk_button_images = True

What I have also noticed is that toolbar button images are not affected by this setting and are always shown. Simply make sure to prepend ../media to the image path in Glade, as Garry Cairns is mentioning in his answer.

You could also try to do it using Python if glade doesn't work for you:

 image = Gtk.Image()
 image.set_from_file("data/media/settings.png")
 image.show()

 button = self.builder.get_object("yourButton")
 button.add(image)

Everything is correct what you did actually...the file name "fb.ico" is the problem! give the full link there for your image. It will show your image in your glade window when you run a code. you know that :-)