It depends: After I advised an author to put her arrangement with her illustrator in writing, she asked me if she should pay for a legal firm's work-for-hire contract template. If you asked me this question, my answer would be the same as the one I gave her: "It depends."
If you're looking for maximum legal protection, pay for and use the template. The downside is that resulting document could be offputting and confusing to your illustrator. It is, after all, written in legalese. So ...
If you're just looking to clarify your arrangement with your illustrator, you might want to write a "memorandum of agreement" in plain English. This won't provide you as much legal protection as the legal template, but it is a written agreement that a court would consider, and it might be a friendlier way of dealing with your illustrator.
What you do want to do is cover the bases, specifying that you are commissioning such-and-such a work product from your illustrator for such-and-such a compensation, in return for which the illustrator will surrender all rights to the created work.
What you don't want to do is nothing (and a verbal agreement with a handshake isn't much better than nothing), even and especially if your illustrator is a relative (as is the case for the person who asked me this question,).--The Publishing Pro.