var watermark = 'path/to/logo.png';
var pathTo = '/stati/nodejs/gm_imagemagick_watermark/path/to/image.jpg'
var pathFrom = pathTo;
gm(pathFrom).
gravity('SouthEast').
draw(['image Over 0,0 0,0 "'+watermark+'"']).
noProfile().
write(pathTo, function (err) {
});