Nodejs test image resizing with mocha

I've used graphicmagick to resize a image in my nodejs app.

The problem is when writing the unit tests, I cant seem to find any direction or examples on this. Does it make sense that I test the image resizing, seeing that I'm using a third-party module? If yes, how could I write a test for my code?

// dependencies
var async = require('async');
var AWS = require('aws-sdk');
var gm = require('gm').subClass({ imageMagick: true });
var util = require('util');

// get reference to S3 client
var s3 = new AWS.S3();

var _800px = {
    width: 800,
    destinationPath: "large"
};

var _500px = {
    width: 500,
    destinationPath: "medium"
};

var _200px = {
    width: 200,
    destinationPath: "small"
};

var _45px = {
    width: 45,
    destinationPath: "thumbnail"
};

var _sizesArray = [_800px, _500px, _200px, _45px];

var len = _sizesArray.length;

// handler for dev environment

exports.GruntHandler = function (filepath) {

    console.log("Path to file is: " + filepath);

    // get the file name
    var srcFile = filepath.split("/").pop();

    var dstnFile = "dst";

    // Infer the image type.
    var typeMatch = srcFile.match(/\.([^.]*)$/);
    if (!typeMatch) {
        console.error('unable to infer image type for key ' + srcFile);
        return;
    }
    var imageType = typeMatch[1];
    if (imageType != "jpg" && imageType != "png") {
        console.log('skipping non-image ' + srcFile);
        return;
    }

    for (var i = 0; i<len; i++) {

        // Transform the image buffer in memory.
        gm(filepath)
            .resize(_sizesArray[i].width)
            .write(dstnFile + "/" + _sizesArray[i].destinationPath + "/" + srcFile, function (err) {
                if (err) {
                    console.log(err);
                }
            });
    }


   console.log(" grunt handler called!");
};