Using nextgen gallery to put full-page background on each page.

Here’s a nifty trick to put a full page self-scaling background on any page using just css, ngg and no additional plugins. Just insert a single pic from your gallery into the post and delete the extra stuff so it looks like this: [singlepic id=259]
Then, add the following to your stylesheet: (If you are already using single nextgen pics anywhere on your site, or want to use images from the media manager you will have to modify the css.)

#content img.ngg-singlepic {
  /* Set rules to fill background  */
  min-height: 100%;
  min-width: 1024px;

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

  /* Set up positioning */
  position: fixed;
  top: 0;
  left: 0;
  z-index:-100000;
}

@media screen and (max-width: 1024px) { /* Set this to your image size */
   #content img.ngg-singlepic {
    left: 50%;
    margin-left: -512px;   /* 50% */
  }
}

https://www.ads-software.com/extend/plugins/nextgen-gallery/