images.js

var createThumb = function(fileObj, readStream, writeStream) {
  gm(readStream, fileObj.name()).resize('256', '256').stream().pipe(writeStream);
};

var createMedium = function(fileObj, readStream, writeStream) {
  gm(readStream, fileObj.name()).resize('800', '800').stream().pipe(writeStream);
};

Images = new FS.Collection("images", {
  stores: [
      new FS.Store.GridFS("thumbs", { transformWrite: createThumb }),
      new FS.Store.GridFS("medium", { transformWrite: createMedium })
  ]
});