What @silverraven0 quoted me as saying earlier in this thread is accurate. You do still own all the rights to your story under our contracts, barring the language of some of our contracts requiring exclusivity (and even then, you own your story, you've just signed a contract giving us the exclusive right to publish that story for a set period of time).
We are actually in the process of adapting several of our Premium novels into comics at the moment (see His Barcode Tattoo, which was originally a novel), and there are lots of plans in the works for other kinds of adaptations (film, TV, games, podcasts, etc.). It's 100% up to creators whether they decide to participate in any of that, but we do put the opportunities out there for the creators who are interested.