Regarding who owns copyrights to images of artworks (as opposed to that of the original works themselves), the statement "if it's older than x years then it's in the public domain" doesn't really apply since museums/galleries tend to keep these reproductive images updated. I'm no expert by any means, but AFAIK this issue has no definitive answer, and museums have asserted de facto rights over them. Google for "zombie copyrights" for more information.

Yeah! That’s another great thing about Tumblr — fact bringers on every subject abound and they often disagree! I guess this is what they mean when they say reproductive rights are a hot topic these days. I’ll just stand out of the line of fire and keep putting up that art…