Jimg is right, you probably need to consult with an attorney. Generally in those situations you're probably okay, especially if it's in Elance's terms of service. If you're really concerned, I'd have them sign an additional contract. I'm not a lawyer, so none of what I say can be taken as legal advice, but this thread might help, specifically this section:
Do I Own the Work I've Paid For?
Not necessarily. If a specially commissioned work doesn't qualify as a "work for hire," you may not own the work -- or even have the exclusive right to use it. While you may have implied license to use it, the scope of your rights will be unclear at best. One way to avoid this situation is to use an appropriate work for hire agreement.
What is a Work for Hire?
One way to acquire rights is by license. With a license, you do not obtain total ownership of the final work, but rather certain limited rights to use it. These limited rights can either be exclusive or nonexclusive. A license can further be defined -- or limited -- by territory, duration, or even media. As a rule, hiring parties prefer to obtain rights on "work for hire" basis (shorthand for "work made for hire"). With a work for hire, the hiring party steps into the shoes of the creator and becomes the author of the work for copyright purposes. With a work for hire, all of the attributes of copyright ownership -- including credit and control -- vest in the hiring party, not the creator.
Important! There are only two situations in which a work for hire can exist. They are: (1) a work created by an "independent contractor," and a (2) "work prepared by an employee" within the scope of her employment.
A. Works Created by Independent Contractors
For a work created by an independent contractor (or freelancer) to qualify as a work for hire, three specific conditions found in the Copyright Act must be meet:
the work must be "specially ordered" or "commissioned." What this means is the independent contractor is paid to create something new (as opposed to being paid for an already existing piece of work); and
prior to commencement of work, both parties must expressly agree in a signed document that the work shall be considered a work made for hire; and
the work must fall within at least one of the following nine narrow statutory categories of commissioned works list in the Copyright Act:
(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).