Here be a sample post with a custom background image. To utilize this “feature” just add the following YAML to a post’s front matter.
background: /images/filename.png
This little bit of YAML makes the assumption that your background image asset is in the /images folder. If you place it somewhere else or are hotlinking from the web, just include the full http(s):// URL. Either way you should have a background image that is tiled.
If you want to set a background image for the entire site just add background: /images/filename.png as a param in your config and BOOM — background images on every page!