gistfile1.rb

$ cat Gemfile
source :rubygems

gem "sinatra"

$ cat config.ru 
require 'rubygems'
require 'bundler'

Bundler.require

require './my_app'
run MyApp

$ cat my_app.rb 
require 'rubygems'
require 'sinatra/base'

class MyApp < Sinatra::Base
        enable :static
        set :public, "public"
	get('/') { ".<img src='a.png'/>." }
end

$ ls public/
a.png  favicon.ico