I've used Woobox to admin contests in the past. www.woobox.com Simple to use good FB integration. They also can provide FanGating to ensure that people like you before completing the contest.
In my opinion frequency is related to the quality of the content. Good content allows you to post more often. Every post should have an image. (Your logo does not count) One of the things I tell my clients is that they should appoint one person to take pictures for a few minutes every day. That way you build up an inventory of relevant images. You can do images of test makeup, progress of a particular room, the day the sign goes up, your crew having pizza after a long day, etc...
I'd suggest posting a couple of times a week until you open. Remember you have a friendly audience interested in the topic; as long as you don't post generic or unrelated crap, you won't burn out your fans.
Once you open I'd do one post per day that you are open start the day before you open for the week. In your case it looks like you're doing a Fri-Sun run for the most part. I'd post Thursday early evening, Friday afternoon, and Saturday afternoon. I'd start changing the nature of the images to patrons having fun/scares. Themes to touch on are date night, scary looking monsters, scaring the girl into the guys arms, fun things to do beyond just the walk-through.
On a bit of a tangent. Get some more content into your website. Do SEO tuning too. Your website doesn't rank on the first page for 'haunted house Plano'
Two cents worth....
Steve Colberg - ScaryTinkerLabs.com