The following code prints HTML with responsive images support. It's ready to use and defines three levels of resolution: 400px, 800px and original image.

Adapt and start using the following code in Ruby (we use Picturefill as polyfill):

image_url = "http://images.rethumb.com/image_coimbra_999x999.jpg";

small_width = 400; # Image width for small resolutions (less than 400px). 
large_width = 800; # Image width for medium resolutions (less than 800px).
image_width = 999; # Image original width (used for resolutions larger than 800px).

puts %Q{

<script src="https://cdnjs.cloudflare.com/ajax/libs/picturefill/2.3.1/picturefill.min.js"></script>

<img src="http://api.rethumb.com/v1/width/#{small_width}/#{image_url}"

     srcset="http://api.rethumb.com/v1/width/#{small_width}/#{image_url} #{small_width}w,
             http://api.rethumb.com/v1/width/#{large_width}/#{image_url} #{large_width}w,
             #{image_url} #{image_width}w"

     sizes="100vw" />

}

Use the following commands to get started:

$ git clone https://github.com/rethumb/rethumb-ruby-examples.git
$ cd rethumb-ruby-examples
$ ruby responsive-images.rb