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" />

}