Several people go to an event (party, wedding, etc.). They all take digital photos and put them on their web sites. (This is the current state of the art.)
Now each of them goes to a public Metagallery installation, and tells it "these are my photos of Robert Sledge's party". Metagallery checks with its administrator that these sites look reasonable, then crawls them grabbing images; it then provides various sorts of thumbnail pages to users combining all the photos it's been given for an event. Full-size images are (optionally) served from the site that originally provided them; thumbnails are generated and cached locally.