Good news! CodePen is a free online tool that you can just ‘use’ – there are paid accounts too but you’re not obligated to sign-up. It’s great for providing live demos of problematic code so that we can help you troubleshoot live in the browser with a real-life sample.
But from your description and code snippets, I’m afraid you aren’t going to be able to move a background-image that currently resides on the body element, so that it appears in front of the table (*). I’m not really sure why you’d really want to do that anyway because it would obscure your website.
However, it’s likely that I’ve misunderstood, so your intentions might become clearer if you provide a diagram/mock-up of what the image is supposed to look like in relation to the layout-table. From there we should be able to offer more helpful suggestions on how you can achieve your goal.
() *maybe with a pseudo element but let’s focus on getting your requirements straight for now