How can an image have a copyright in a book when the image is public domain?

I recently started doing business on Zazzle and decided that one of the best ways to make some extra cash would be to try and sell vintage Public Domain images. I started my research on the web, which produced a large quantity of low res images. No good for print.

The idea for this particular store was not waste too much time designing and still make some money, so when my next thought was to go to the library and scan these images - it was quickly squashed.

Then of course I remembered that I have a slew of Dover Books some where Permission Free Designs, but most where Royalty Free Designs. What confused me was that most if not all the images where in the Public Domain. Which brings us to the question "How can an image have a copyright in a book when the image is public domain?" With some more research I found The Stanford University Libraries and Academic Information Resources to be immensely useful.