Who owns intellectual property?

I recommend that you consult an attorney to discuss your case in full before relying on any advice below.  I also like to limit my advice only to the facts provided.  Additional facts shared with your attorney may reveal a more appropriate strategy. 

I do discuss some good news for you; however, they likely own the copyright to the illustrations under section 201(b) of the Copyright Act as a "Work Made for Hire."  Section 101 defines a "work made for hire" as "a work prepared by an employee within the scope of his or her employment; or a work specially ordered or commissioned for use as a contribution to a collective work, as part of a motion picture . . .if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire."  Under a conservative analysis, you were an employee working within the scope of employment even though there was no written agreement and no compensation.  

Your jurisdiction's law will determine if you truly were an employee in this situation; however, most jurisdictions do not require a writing to establish employment.  Further, the fact that you did not receive payment does not always prevent a court from finding a valid implied contract of employment or maybe even a unilateral employment contract.  Where terms are missing in a contract, specifically the price, your jurisdiction may imply a "fair wage" for the work provided.  This is likely since you already performed your half of the implied/unilateral agreement.

The second half of the "work for hire" definition specifically requiring a writting will not be relevant where you meet the criteria of the first part of the definition.  Additionally, section 202 of the Copyright Act provides that physical control of the work is separate from the copyright.  

Therefore, I suggest you transfer the physical control of the works to them by way of registered mail (forcing them to sign for receipt).  Include in the mailing a letter stating your position that "This delivery completes our original agreement for my contribution of opening sequence illustrations to your collective work tentatively titled 'blah blah blah'.  This transfer does not forfeit my copyrights in the illustrations nor my rights to be listed in the credits to the completed movie, any revision thereof, or any later collective work in the same series.  Our original agreement to these terms arose on X date when {state all the truthful facts you can remember}.  The fair market value of my services is calculated to be $XX,XXX and payment is due within 30 days of the receipt of this letter.  Late payment fees will accrue as X% calculated daily.  In the event that legal action is necessary to recover my fee all related legal fees will be paid by you.  You agree to these terms by the acceptance of delivery of these works and may only rescind your agreement by return of the undamaged work to this address ________ within 5 business days."  

This should preserve your claim to the copyright as a "contribution to a collective work" under section 201(c) of the Copyright act, establish a way for a judge/jury to calculate damages if you eventually have to go to court, force them to sign their name to your version of the facts (which would be used in a later court pleading), possibly obtain payment for your efforts immediately and provide a calculation for larger payment if they does not comply with your terms, and give you further recourse if they fail to give you attribution when they finish themovie.

The risk of this course of action would be that they do not pay.  You would then be forced to sue to recover either the money or illustrations and they do have a solid foundation for their own claim to them.  Alternatively, they could pay with a similar letter that you would need to read carefully before accepting or depositing the money in your account.