How to position a background image in reveal.js?

This article helped to find the solution. You can add a style to the html element for the current section using

data-state="something"

and of course you can use that style for tweaking your css. So I used this html:

<section data-background="myImage.jpg" data-state="img-right">
    // content
</section>

and the following css:

html.img-right div.slide-background.present {
    background-position: right;
}

Based on the "new" solution, in the css file you can include something like this, modifying the size and position values will adapt it to the right size and location on the slide.

body {
  background-image: url(image1.png),url(image2.png), url(image3.png), url(image4.png);
  background-size: 15%, 15%, 15%, 15%;
  background-repeat: no-repeat, no-repeat, no-repeat, no-repeat;
  background-position: 95% 96%, 95% 3%, 3% 3%, 3% 95%;
}