Use GM Module (Graphics Magick) for Node.js to center an image on a transparent background.

graphics_magick_center_image_on_transparent_background.js

var gm = require('gm');

var canvasWidth = 248;
var canvasHeight = 389;

gm(__dirname + '/original.jpg').size(function(error, size) {
  if (error) {
    console.error(error);
  } else {
    // current image size
    var imageWidth = size.width;
    var imageHeight = size.height;
    // center placement
    var x = (canvasWidth / 2) - (imageWidth / 2);
    var y = (canvasHeight / 2) - (imageHeight / 2);
    console.log(x, y);
    this.background('transparent')
    .resize(imageWidth, imageHeight)
    .gravity('Center')
    .extent(canvasWidth, canvasHeight)
    .write(__dirname + '/test.jpg', function(error) {
      if (error) {
        console.error(error);
      } else {
        console.log(this.outname);
      };
    })
  }
});