More about copyright infringement than royalties.
I am not a lawyer but, info i dug up from a google search of "mona lisa copyright" seems to indicate that any works made prior to the existence of any copyright laws are obviously not under copyright protection.
This wikipedia entry on "public domain"
http://en.wikipedia.org/wiki/Wikipedia:Public_domain list various copyright terms for different types of works.
Seems to be safe, works 170 years old or more are almost certainly in the public domain. That would cover the cases that say a copyright expires 70 years after the death of the author. But I think there are some copyright extension laws out there... i recall hearing that Disney was trying to extend a lot of its copyrights for characters like Micky Mouse. Imagine a public domain Micky Mouse?