Definitely add screenshots.
Playing SC2 campaigns nowadays is so ass-backwards that most people just won't bother downloading it if there's no proof of quality.
Additionally, you should never sacrifice one aspect of your project over another. If you need to spend more time on it to improve it, then do that. It'll make your project look much, much better to the people who have never seen it before, rather than leaving a negative impression with the crowd exposed to what many would call an unfinished or unpolished project.