Image as background for custom create-content form

I'm trying to use an image as the background for the create-content-form for a content type I've just created. In order to do this, I think I need to create a table with an image as the background first, then place the code for the create-content input-form inside that table. I should probably do all of this inside a block. As for the first step of making an image the background for a table, do I do that via the my theme's style.css file? Any other thoughts on how I could do this? I'm very weak on html and css, but am learning.