How can I implement an Add to Collection function on my Image Gallery to be downloaded later?

I would skip cookies and use the awesome script store.js

Just kick out a data-lightbox attribute on the “Add to Lightbox” link with the image ID and (assuming you’re using jQuery) it’s this simple:

$( document.body ).on( "click", "a[data-lightbox]",
    function ( ev ) {
        ev.preventDefault();
        var id = $( this ).data( "lightbox" ), list = store.get( "lightbox" );
        if ( list ) {
            if ( $.inArray( id, list ) === -1 )
                list.push( id );    
        } else {
            list = [ id ];
        }

        store.set( "lightbox", list );
    }
);

And to download, just process the list & redirect to your server-side handler:

$( ".download" ).click(
    function ( ev ) {
        ev.preventDefault();
        var list = store.get( "lightbox" );
        if ( list ) {
            var href = window.location.href,
                args = ( href.indexOf( "?" ) !== -1 ? "&" : "?" ) + "download=" + list.join( "," );

            window.location = href + args;
        }
    }
);

I’ve made the assumption that your handler accepts a comma-separated list of IDs in $_GET['download'] for any given URL.