You may have paid for and you may possess the original art work, but the 17-year-old who did the illustrations has retained all rights in the work without a signed written agreement to the contrary. These rights include, but are not limited to, the
exclusive right to make copies, authorize others to make copies, make derivative works, sell or market the work, display the work, sue an infringer.
The 17-year-old can agree to give you all rights in the work (an assignment of her rights) or she can agree to give you some of the rights in the work (a license for a particular use).
Copyright in any work is automatic upon creation of the work. Generally the copyright owner will be the individual who created the work. There are exceptions.
One exception is when a work is created by an employee within the scope of the employee's employment. This is called a work made for hire and the copyright in a work created as a work made for hire is held by the employer. Another work made for hire situation is when there is a written contract between an artist/author and an individual who commissions a work from an artist/author. In this situation the work that is created must fall within one of nine categories (see Section 101 of the Copyright Act at
http://www.copyright.gov) and again it must be a written contract.
Although minors can enter into contracts, they cannot under most circumstances be held legally liable if they fail to meet the terms of the contract. It would be wise, therefore, for you to have both the girl and the girl's parents sign any agreement that is drawn up that transfers (all or some) the copyrights in the illustrations to you.
It is additionally wise to have any agreement reviewed by an attorney prior to signing, to ensure that it covers what it needs to cover, so that there are no surprises for either you or the girl later on should your illustrated book earn for you a lot of money.
Good luck.
