How can I create a simple JPEG "COMING SOON" page?

I am by no means a wordpress expert, but I do have photoshop, and I am capable of creating a JPEG that I’d like to be the “COMING SOON” page for my site – http://www.crowdcavalry.com – however, I have no clue how to set a JPEG as a landing page. (some useful information: I’m running the latest version of WordPress and I’m using the Custom Community theme).

As of now, the site is in its pre-launch phase. I’d prefer it if site visitors couldn’t see my efforts to populate the various pages (this strikes me as rather unprofessional).

All help is much appreciated. Thank you in advance.