How we serve static graphics that can't fit in our CMS using sinatra. We have a layout file that pulls in the header and footer from the main site (and uses content_for to populate the page title and stuff like that.) You are never more than 15 minutes away from implementing this.

app.rb

require 'sinatra'
require 'sinatra/content_for'

set :static, true

Dir.glob("#{:views}/*").each do |file|
  next if file =~ /layout.*/
  url = File.basename(file, ".erb")
  get "/#{url}" do
    erb url.to_sym
  end
end

not_found do
  erb :'404', :layout => :layout_basic
end