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.

"An important wrinkle to understand about public domain material is that collections of it may be protected by copyright. If, for example, someone has collected public domain images in a book or at a website, the collection as a whole may be protected, even though individual images are not protected. You are free to copy and use individual images but copying and distributing the complete collection may infringe what is known as the "collective works" copyright. Collections of public domain material will be protected if the person who created it has used creativity in the choices and organization of the public domain material. This usually involves some unique selection process, for example, a poetry scholar compiling a book, The Greatest Poems of e.e. cummings."

Clip Art Compilations

Generally clip art is sold in books, CD-ROM bundles or from websites, and is often offered as "copyright-free." The term "copyright-free" is usually a misnomer, and actually refers to either royalty-free artwork or work in the public domain. Keep in mind that much of the artwork advertised as copyright-free is actually royalty-free artwork, which is protected by copyright. Your rights and limitations to use such artwork are expressed in the artwork packaging or in the shrink-wrap agreement or license that accompanies the artwork. If the artwork is in the public domain you are free to copy items without restriction. However, even if the artwork is in the public domain, the complete collection may not be reproduced and sold as a clip art collection since that may infringe the unique manner in which the art is collected (known as a compilation or collective work copyright).

So, as far as I can tell I can use these images in my vintage shop as long as I am not selling them as a clip art collection. Thoughts anyone?