Well, by law, once you take a picture, you already own it and own the rights. No one else can use it...legally.
I'm not 100% clear on this, but you can PM them, saying that they own the picture. As long as they have your consent, they should be good. Giving consent and rights isn't very difficult, I've done it. Such as, if you submit a photo for something, they will have a banner or caption saying "By uploading, you are agreeing to send the rights of the photo to ____". It's usually that easy.