There are more than a dozen jigsaw sets on this site, several of them running on the same engine with different pictures. The piece count is the number offered on the selection screen, and it is not the number that decides how long a board takes.
The subject does. Consider three boards of a hundred pieces each. A parrot photographed close gives strong colour separation — the wing pieces, the body pieces and the background pieces sort into three piles almost immediately, and the board resolves as three small puzzles rather than one large one. A tiger gives you stripes, which defeat shape matching entirely: every piece looks like every other piece, and the face is the only region where individual pieces are distinguishable. A penguin against snow is worse still, because most pieces carry no information at all — they are white, and the only ones worth placing early are the few that hold a black-white boundary.
This is why the silhouette set is the hardest jigsaw on the site despite having modest piece counts. Reducing an elephant to a dark shape against a light background removes the internal detail entirely, and every decision comes down to the outline.
The practical consequence is that sorting is not a preliminary to the puzzle — it is the puzzle. A minute spent grouping by colour or by focus buys back three at the board, and on the harder subjects it is the difference between finishing and giving up.
If you want a jigsaw that will take a while, do not raise the piece count. Choose a picture with less to navigate by.
