In order for you to retain copyright ownership of the cover with a work for hire situation, you need to have a signed agreement before the work begins. However, since you do not have one, you have two options:
1) Unhire the person from working for you and get a written agreement signed defining ownership, then rehire the individual. Make sure that the individual you rehire starts work on the cover over again and does not continue on the old work.
2) Make a written agreement now. However, in some places, having a written agreement after the work starts is this will still not enough. However, this is better than not having a written agreement at all.
In addition, in order to have a work for hire situation, the following conditions must be met:
1. You must pay the independent contractor to create something new, not to create something that already exists.
2. Before work is done, both parties must agree that the work will be for hire (and that the independent contractor does not retain copyright to his work). A writing is required but again, in your case, you should do it as soon as possible if going back is not possible.
3. The work must fall under the categories of copyrightable works under the Copyright Act (which it does under number 9 below).
(1) a translation, (2) a contribution to a motion picture or other audiovisual work, (3) a contribution to a collective work (such as a magazine), (4) as an atlas, (5) as a compilation, (6) as an instructional text, (7) as a test, (8) as answer material for a test, (9) or a supplementary work (i.e., "a secondary adjunct to a work by another author" such as a foreword, afterword, chart, illustration, editorial note, bibliography, appendix and index).