I'd like to create some original art using images from art history. Let's keep the question widely applicable and say that I might be interested in anything from old paintings and engravings to pen and ink magazine art to early photography, but all of it would be pre-1900s. Many of the images I'm interested in go back much further, all the way to the late 1500s. I live in the United States, but might want to source images from overseas.
Presumably the copyright holders for these images are all long expired, and if I've got things right because they predated the copyright law enacted in the early 1900s these images should all be in the public domain.
A quick google and a perusal of some art books tells me it isn't that simple. Image archives, whether online or in photo book form, generally appear to make a copyright claim on the images they contain. This makes my want to get a generally more robust idea of what I can and can't do with art historical and early photographic images before I go any further.
My question is, what should I know about copyright law and using images that were created prior to the 1900s? What is the best legal practice for reusing images like that in original artwork?