Themes, blocks, and content ... Oh my! I am a newbie to Drupal/Acquia Dev Desktop. Please excuse me if this is a dumb question.

I'm trying to grasp the relationship between themes (the one I chose is Corked Screwer), blocks, and content. In Corked Screwer there are images on the top half of the page/node and my content is at the bottom. 

Am I understanding correctly that Content/main page content is where my typed-in content is displayed? I can control this with Visibility Settings/Content Types, etc. Yes?

How do I go about customizing the top half of the page to replace the default images with my own? Can you point me to a good video tutorial?

Thanks!
Yes, the Main page content block is where your content will render. Yes those visibility settings control on what page/content type/role will show that block content area. Typically, you want to leave this block alone since it's the main presentation of your content (what's the point of having a page if it doesn't have any content?).

It looks like the CorkedScrewer's default home page is more a demo template to demonstrate its block regions; you can't replace the images without overwriting the existing ones in its images folder.  It can vary based on the theme developer, but usually you can control a theme's slideshow images from its settings page in /admin/appearance.