Recomended default CSS

img.evonabackground  {
  z-index: -999;
  /* Set rules to fill background */
  min-height: 100%;
  min-width: 1920px;

  /* Set up proportionate scaling */
  width: 100%;
  height: auto;

  /* Set up positioning */
  position: fixed;
  top: 0;
		}
@media screen and (max-width: 1024px) {
img.evonabackground  {
    left: 50%;
    margin-left: -960px;  /* 50% */
}
}

I set this as my default css while using this plugin, should be useful for anyone else too.

https://wordpress.org/plugins/random-backgrounds/