Resize and Upload Image Flow in NodeJS

You need to remember that node.js code runs asynchronously. In your original code, the knox code is running before image.resize has completed (the callback is used to tell you when the operation has completed, not just to handle errors). Node won't wait for the callback and just continues to execute code in your function. You also need to be careful of using anonymous callbacks in for loops without creating a closure.

In general you want to use the callbacks to control program flow like the code below so that you only do the following action when the preceding action has completed.

var src = name + '.jpg';

for (var i = sizes.length - 1; i >= 0; i--) {
    var k = i;

    var dest = sizes[k] + '.jpg';
    var s3   = sizes[k] + '.jpg';
    resizeAndPut(src, dest, s3, sizes[k]);
}
fs.unlink(src);​  /* delete the source file now */

var resizeAndPut = function (src, dest, s3, size) {
    easyimage.resize(
    {
        src:    src,
        dst:    dest,
        width:  size,
        height: size
    }, function(err, image) {
            if (err) throw err;
            knox.putFile(dest, s3, function(err, res) {  /* does image contain the path?, if so, might want to use image.path or the like instead of dest here */
                if (err) throw err;
                fs.unlink(dest); /* delete the local file*/
            });

    });
};