If someone anonymously posted them to alt.binaries.e-book.technical they could never be taken down and would be available to everyone forever.

If it's Bally/Williams artwork then you need to get the Licensing from PPS

Yep, depends on the title. I recreated the Rollergames insert from scratch. It was just bold Arial Font and some geometric shapes. Did this without scans. So there is no artwork to license. The team logos for the middle six inserts are owned by the Rollergames people and not Williams.

For Taxi you could openly distribute the text inserts but not the cars or faces or people.

Also it's probable under fair use that you could probably share the artwork scans with people who own a Taxi and plan on restoring it as long as you did not sell them decals or sell them the files. Would be somewhat similar to ROM files. You both own the game, you both have license to own the artwork and rom files, you both have the right to restore your games.