"I understand the text is copyrighted but are the images?"
If the images are covered by copyright, then yes. If not, then no. I know this may not be the most helpful of answers, but that's sort of the way it is...
"Can a person or organization copyright a dead artists work from over a century ago?"
No. Only the creator of a work gets a copyright (unless its a work-for-hire). The creator can sell or license the copyright, in whole or in part. Once a piece moves into the public domain, it is basically impossible to recover the copyright protections. A person may be able to get a copyright on a derivative work based on a public domain work -- for example, if I took a copy of the Mona Lisa, gave her a mustache and an new hairdo, I could get a copyright on the derivative work, but I would still NOT have a copyright on the underlying work -- therefore I could prevent someone else from using my "new" version of the Mona Lisa, but I could not prevent someone else from starting out with the original Mona Lisa and creating their own "new" Mona Lisa.
"Do museums hold some rights over whats in their exibits?"
Only if the works are still covered under copyright and the museum has obtained or licensed those rights. If the museum has a particular display or frame that they use, the combination may be protected by copyright, but the underlying work -- if it is public domain -- would not be.
"How do I find out what is public domain and what is not?"
There really isn't an easy way. If a work was published before 1911, it is certainly public domain, as any copyright would have expired by now (actually, I think the year is closer to 1920, but I'm not 100% sure -- you could do a Google search and probably find out an exact date). If a work is published after that date, it gets complicated -- it has to do with whether a work was copyrighted properly in the first place, whether the term of copyright was renewed, etc.
Bascially, if you are unsure, there are three ways to determine if a work is public domain -- contact the creator and find out (or contact whoever he licensed or sold the rights to), hire a lawyer or firm (I'm sure there are firms that do this kind of stuff out there) to do a search, or use the material and wait to see if you get sued.
I don't really recommend the third option!
