Can We Use Illustration for Cover of Book?

Dear Rich: I run a non-profit educational website called Simply Charly that explores history's greatest movers and shakers. And we're planning to publish a series of books using many of the illustrations we own on the covers of the books. I was wondering if we'll run into some copyright issues using our own illustrations. You need to look at the paperwork associated with the acquisition of the images. As a general rule, if you: (1) employed the artist who created the covers, (2) hired the artist and had him or her sign a work made for hire agreement, or (3) paid the artist and received an assignment of copyright, than you can use the illustrations for covers --  that is, you own the copyright and are free to use the works as you please. (By the way, no written agreement is needed if the artwork was created by employees.) That said, assignments and work made for hire agreements may have additional requirements that affect your rights. Theses agreements may transfer copyright but may condition the transfer on the payment of fees or royalties for certain uses --- for example, $1,000 will be paid if the illustration is used as a cover.
If you acquired a license ... If you acquired image rights under a license, then you are "renting" the images and would probably need to return to the artist for additional permission to use the artwork on the cover of a book. The license agreement probably explains the limitations.
P.S. Dept. We like your books and covers ... if only we weren't so busy with Words With Friends.