Gemfile

source "https://rubygems.org"

gem "sinatra"
gem "puma"

lorempixel.rb

require 'sinatra'

set :port, ENV['PORT'] || 5000

get /(.+)/ do
  p = params[:captures].split(/\/|\./)

  # Available params: width/height/category.format
  width = p.empty? ? 600 : p.shift
  height = p.empty? ? 400 : p.shift
  category = p.empty? ? "**" : p.shift
  format = p.empty? ? 'jpeg' : p.shift

  # puts [width, height, category, format]
  
  # TODO: crop the image

  status 200
  headers "Content-Type" => "image/#{format}"
  File.read Dir["img/#{category}/*.#{format}"].shuffle.first
end

Setup:

gem install sinatra puma #or bundle install
ruby lorempixel.rb

Go to http://localhost:5000/200/300