insert specific background to single post

Occasionally, I’d like to write a post, with background image specific to the post. I do understand (only from the codex) how to edit the css, and templates. I think that this would require a conditional query for an image, each time I do this. Can anyone help write this?

My idea: Maybe if I called the image “new post bkgd” and overwrote it with each new upload, then I could query the same title, and actually get a different image? (oh – but then the archives would have a broken link, right? – is there a way to put the image IN the post, without a link?) Is there a way to query some sort of clue in the post content itself?

Also, there are no plugins that I can find on google or wordpress, using keywords – layer, image, background, or insert.)