Captcha For Submitting Posts (Not Comments)

I am looking for a solution that would allow me to require certain roles to complete a Captcha before submitting a post.

I allow users to sign-up and submit their own posts as Contributors through the backend (wp-admin). I want to be able to force them to pass a Captcha for each post that they try to submit for review.

Is there a plugin that will handle this? Is it something that can be coded into WordPress without really extensive coding?

Thanks for any help you can offer with this.