I'm currently trying to get a single image (website under construction) to cover the entire Tumblr website I'm working on, temporarily, without losing any of the underlying HTML and CSS code. Additionally, I'm not sure where to put the image itself (head, body, etc) to get it to cover up the menus and other links that are above the content without changing the structure.
There are many ways to do this.
The most efficient way would be to have a new page with just the image and put a temporary redirect on your DNS entry if that is accessible to you. If you do not have access you can add a redirect into the head tag. (If you are looking to eventually have good SEO(Search Engine Optimization) you should be wary how you perform this redirect. Hosting the site before its ready may negatively impact rankings.
Add the image to a div or the body tag as the background with 100% height and give it a large z-score to raise it above the rest of the content.