i will write it one last time and pin it. how to create multiplayer puzzle:
https://www.jigsawexplorer.com/create-a-custom-jigsaw-puzzle/
you can upload an image to forum, but not post it. copy the image address from reply window. i use 200-300 pieces, depending on how detailed the image is. if there are parts of image where there would be too many similar pieces, then use less of them
when you create the puzzle, click "try it", but dont copy the link, it's a single player link. click the highlighted button on image above and you will get a multiplayer link