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 Node.js (we use Picturefill as polyfill):

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

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

console.log(
    '<script src="https://cdnjs.cloudflare.com/ajax/libs/picturefill/2.3.1/picturefill.min.js"></script>\n'+
    '\n'+
    '<img src="http://api.rethumb.com/v1/width/'+small_width+'/'+image_url+'"\n'+
    '\n'+
    '     srcset="http://api.rethumb.com/v1/width/'+small_width+'/'+image_url+' '+small_width+'w,\n'+
    '             http://api.rethumb.com/v1/width/'+large_width+'/'+image_url+' '+large_width+'w,\n'+
    '             '+image_url+' '+image_width+'w"\n'+
    '\n'+
    '     sizes="100vw" />'
);

Use the following commands to get started:

$ git clone https://github.com/rethumb/rethumb-nodejs-examples.git
$ cd rethumb-nodejs-examples
$ node responsive-images.js