Check the t&c's for the competition, they' usually have an entrant sign away any rights to the image, your model would have signed to say this was OK. You can go ask for retrospective credit, but to push for more than that will probably do you more harm than good.
If the organisation have no rights to the picture they'll likely disqualify your model and then you're in a worse place.
Your best option is to get it built into your marketing and make something from it rather than worry about what might have been. Blog it, Facebook it, print it large and show it. Take pride in the success rather than spoiling it for everyone.