Hmm using multer seams to be a good choice. On the other hand the default implementations don't seam like rocket science either.
Since gridfs is up for debate the max size is 16MBs per File if I read correctly. So I expect you aim for 300kb - 700kb per image ?
I personally tend to only store the path and not the content in the database which makes gridfs obsolete. This is mainly because I like that the images are on the file system and not in the database. It's more work to delete them but still you don't bloat your database with binary data.
But this is taste I guess.
I have to be honest I don't work with angular anymore but I guess you're working with $resource in angular so you could actually use the filereader object and store the base64 encoded images in the localstorage {name: "blablub", content: "Base64..." } and iterate them so if a connection los is happening you could automatically retry ? like an upload queue ?
or something like this: stackoverflow.com/questions/21115771/angularjs-up… ?
Sorry that I can't be really of help I don't see a "best way" I just see constraints and options.