gallery.js

var Gallery = React.createClass({
    _handleUpload(files) { //this function is called whenever a file was dropped in your dropzone
        _.each(files, function(file) {
            file.owner = Meteor.userId(); //before upload also save the owner of that file
            Images.insert(file, function(err, fileObj) {
                if (err) {
                    console.log(err); //in case there is an error, log it to the console
                } else {
                    //the image upload is done successfully.
                    //you can use this callback to add the id of your file into another collection
                    //for this you can use fileObj._id to get the id of the file
                }
            });
        });
    },

    render() {
        return(
            <div>
                <Dropzone onDrop={this._handleUpload}>
                    <div>Try dropping some files here, or click to select files to upload.</div>
                </Dropzone>
            </div>
        )
    }
});