Full screen background images are a nice feature for a number of WordPress themes. It’s also one of the few theme modifications that can be handled well through a plugin.
The implementations I’ve seen generally use the jQuery script backstretch.js- which handles responsive layouts and large images elegantly.
If you’re not a theme developer and just want to add a full screen background to your site, I’d suggest trying out one of these plugins:
If you are a developer and want learn how to add this feature to a theme, read on.