Why not make Tags and Albums be the same thing?  Users tag their photos with names/places/vacations, upload to Piwigo and they would magically (without having to do searches and bounce through the caddie)
appear in the right Category/Album.  Sure some Albums would get uselessly large for popular tags (like one for your kid's/cat's name).

An alternative would be virtual folders that are populated with all photos that match a set of tags.

I've seen the php tweak to convert IPTC tags into piwigo tags, would similar work for associating an image with an album/category?  Maybe, not all tags.  Maybe a user defined subset?

Something like:
$conf['tag_album_map'] = array(
     Jack => Kids,
     Jill => Kids,
     June => Summer,
    July => Summer,
    August => Summer,
    hike => Trips,
    zoo => Trips,
    lake => Trips,
);

So each photo with any of the above tags ends up in /Kids, /Summer, and/or /Trips

Then any tag not in the map doesn't create any album association.