gistfile1.rb

# This assumes you have a folder structure like the following:
# - App
#   - myapp.rb
#   - secure
#     - foo.jpg

require 'sinatra'

get '/' do
  'Hello, World! <img src="/image/foo.jpg" />'
end

get '/image/:img' do
  send_file File.join('secure', params[:img]), :type => 'image/jpeg', :disposition => 'inline'
end