ImageMagick

imagemagick-nodejs.js

// Require our module dependencies
var exec = require('child_process').exec;

// Create command array to invoke ImageMagick composite where
// -dissolve is the amount of transparency for the watermark
// -gravity tells how to align images of varying size
// -quality is the image quality of the JPEG (not required if producing PNG)
var command = [
    'composite',
    '-dissolve', '50%',
    '-gravity', 'southwest',
    '-geometry', '+50+100', 
    '-quality', 100,
    '-colorspace', 'gray',
    'watermark.png',
    'image.jpg',
    'out/result.jpg'
];

// Join command array by a space character and then execute command
exec(command.join(' '), function(err, stdout, stderr) {
    // Do stuff with result here
    if (err) console.log(err);
});