I am an artist and I have had to deal with copyright issues in the past. It is really very, very simple.
Whomever is the initial creator of any artistic product (painting, book, screenplay, arcade art, ect) is the copyright holder of that piece of artwork. It does NOT have to be registered to be "copyrighted". It is inherintly the intellectual property of the intitial creator no matter what. That means if someone took a screenplay and rewrote everything except one page, the initial creator STILL owns the copyright because it was their initial executed concept. You cannot copyright an "idea", it must be an executed piece of artwork.
So yeah, all that cool arcade art out there is copyrighted but unless you are trying to resell it, no big deal for your personal use.
The biggest deal in arcade art legality is without a doubt--Ms Pacman artwork. Namco has all the rights to Ms Pac/Pacman and Galaga artwork. Only twobits.com sells this legally, as far as I know. It is even hard to find scans and vectorized versions of Ms Pac artwork. Other printers can produce Ms Pac art but it is illegal for them to sell it, technically speaking.
In fact, a client gave me a full set of Ms Pac art to install on a Ms Pac cabinet. Everything, kick plate, both full sides on vinyl and a nice CPO. Because he buys in bulk, he printer sells him full sets for $60!!! But we are talking volume discounts here and the source cannot be revealed, of course.