I have bene using node-imagemagick for a few days now and have come to realize that it has bugs.
There are about 100 forks of it, some of which fix some of the issues i have come across, but it is hard to figure out which fork i should use.
I once was in your position and after getting really frustrated with modules that had bugs or weird APIs I started using imagemagic directly by spawning a child process. Node.js is pretty good at this so it's actually not that hard.
var spawn = require('child_process').spawn;
var args = ['-ping', 'tree.gif' ];
var composite = spawn('identify', args);
It's also great because you can just use the imagemagic documentation.