app.rb

require 'sinatra/base'

IMAGES = [
  { title: "matrix", url: "http://i0.kym-cdn.com/entries/icons/original/000/009/889/Morpheus2.jpg" },
  { title: "google", url: "http://i0.kym-cdn.com/news_feeds/icons/original/000/006/501/google-search.jpg" },
  { title: "skywalking", url: "http://i2.kym-cdn.com/photos/images/newsfeed/000/295/090/9de.jpg" }
]

class App < Sinatra::Base
  get '/images' do
    @images = IMAGES
  end

  get '/images/:index' do |index|
    @image = IMAGE[index]
  end
end

error

macbookpro: ~/code/sinatra-app                                                                 
→ ls -la
total 16
drwxr-xr-x   4 jacob  staff  136 29 Dec 15:36 .
drwxr-xr-x  12 jacob  staff  408 29 Dec 15:10 ..
-rw-r--r--   1 jacob  staff  491 29 Dec 16:06 app.rb
-rw-r--r--   1 jacob  staff   25 29 Dec 15:14 config.ru
                                                                                                     
macbookpro: ~/code/sinatra-app                                                                 
→ rackup         
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:36:in `gem_original_require': ./app.rb:4: odd number list for Hash (SyntaxError)
  { title: "matrix", url: "http://i0.kym...
          ^
./app.rb:4: syntax error, unexpected ':', expecting '}'
  { title: "matrix", url: "http://i0.kym...
          ^
./app.rb:4: syntax error, unexpected ',', expecting $end
  { title: "matrix", url: "http://i0.kym-cdn.com/e...
                    ^
  from /Library/Ruby/Site/1.8/rubygems/custom_require.rb:36:in `require'
	from /Users/jacob/code/sinatra-app/config.ru:2
	from /Users/jacob/.rvm/gems/ruby-1.9.2-p320/gems/rack-1.4.1/lib/rack/builder.rb:51:in `instance_eval'
	from /Users/jacob/.rvm/gems/ruby-1.9.2-p320/gems/rack-1.4.1/lib/rack/builder.rb:51:in `initialize'
	from /Users/jacob/code/sinatra-app/config.ru:1:in `new'
	from /Users/jacob/code/sinatra-app/config.ru:1

ruby -rubygems app.rb

macbookpro: ~/code/sinatra-app                                                                 
→ ruby -rubygems app.rb
/Users/jacob/.rvm/rubies/ruby-1.9.2-p320/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require': no such file to load -- sinatra (LoadError)
  from /Users/jacob/.rvm/rubies/ruby-1.9.2-p320/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from app.rb:1:in `<main>'

Hey, here is the working version:
https://gist.github.com/4405233