Basic Wordpress Theme

I would like to have a very basic Wordpress theme created. I write enough HTML to effectively use the capabilities within Wordpress so I'm looking for someone to create a "shell" of a design. Ideally, the design would be a PNG file with "text area" in the middle. I could then edit the text area with anything I want through the Wordpress editor. My goal is to have this shell be editable by me designing PNG files that are the same dimensions. The nav menu would stay in the same location, the only thing I would change is the shell image around it. The design will change, but I'm picturing a shell like this: [login to view URL], but where I can replace the PNG background to change the design. Does this make sense? I'm open to suggestions if you have a better process to make this happen so I can change the basic shell/look of the site. Thanks! Scott