I have written some middleware for uploading an Avatar, like this:

var gm = require('gm'),
    mkdirp = require('mkdirp'),
    fs = require('fs');

uploadAvatar = function(req, res, next) {
  var img, path, user;
  if (req.files.avatar_image) {
    user = req.user;
    img = req.files.avatar_image;
    path = __dirname + "/../../public/avatar/" + (user.name.parameterize()) + ".png";
    mkdirp.sync(__dirname + "/../../public/avatar/");
    fs.createReadStream(img.path).pipe(fs.createWriteStream(path));
    gm(path).resize(250, 250).autoOrient().quality(90).write(path, function(err) {
      if (err != null) {
        req.flash('error', err);
      } else {
        user.avatar = "/avatar/" + (user.name.parameterize()) + ".png";
        user.save(function(err) {
          if (err != null) {
            req.flash('error', err);
          }
          next();
        });
      }
    });
  } else {
    next();
  }
};

// Usage
app.post('/upload', ensureAuthenticated, uploadAvatar, handleUpload);

When I now try to upload an image, node crashed with the incredibly helpful error message:

events.js:72
    throw er; // Unhandled 'error' event
          ^
Error: ENOENT, open '/tmp/1126846a248af5c584770b07de649f9b.png'

I have tried copying the file before using gm on it, too. I suspect that express deletes the file before I can even touch it, as a "security".

Can anyone help me here?

EDIT: Full source

First of all copy temp file to your avatar directory, and second make sure that you have permissions on avatar as well as temp directory and files.

As well piping read to write stream is not sync operation, and you will try to pipe straight after initialising read handle, that might lead to some problems.
You need to add events and wait till file get copied:

var complete = function(err) {
  if (!err) {
    // process your gm
  } else {
    console.log(err);
  }
}

var read = fs.createReadStream(sourcePath);
read.on('error', function(err) {
  complete(err);
});

var write = fs.createWriteStream(targetPath);
write.on('error', function(err) {
  complete(err);
});
write.on('close', function() {
  complete();
});
read.pipe(write);