viewer.rb

#!/usr/bin/env ruby

require 'gtk3'

fname = ARGV[0]

def window(&b)
  w = Gtk::Window.new
  proc { w.instance_eval(&b) }
end

image = Gtk::Image.new(:file => fname)

app = window do
  fixed = Gtk::Fixed.new
  add fixed

  button = Gtk::Button.new :label => "Quit"
  button.set_size_request 80, 30
  button.signal_connect "clicked" do
    Gtk.main_quit
  end

  fixed.put button, 50, 50

  image = Gtk::Image.new(:file => fname)
  fixed.put image, 10, 10

  Gtk.timeout_add 200 do
    puts "here"
    image.set_file(fname)
    image.show
  end

  show_all
end

Gtk.init
app.call
Gtk.main