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?
$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.
mistic100 is working on a feature called "Smart Albums" which will automatically fill albums based on any criteria. The criteria can be photos tagged with "lake" or "zoo" or "hike".