Get a lawyer to draw up a frame agreement for the corporation. No matter how much you trust each other, you need to get some documents in place that protect the individual interests of each party. Don't allow loopholes to exist that permit one party to screw over the other, no matter how convinced you are it would never happen.
Trust me on this.
For such a small contribution though the whole thing sounds rather silly. Better to pay something upfront and/or ask for pro bono.
The woman who designed the Nike Swoosh was simply paid. Several years later the founder of Nike gave her stock to show his gratitude for helping define the brand.
Show anyone the swoosh and they either associate it with "Just Do It" (again, Nike's trademark) or Nike itself.
You might as well claim that Apple's startup chime is just another beep.