I'm seeing that a lot of sites (moz.com,hubspot.com) are using a background image as a layout. How do I add a single background image easily onto WordPress? I also want to add the layout here: http://moz.com/blog onto my blog. How do I do this?
This assumes that you want to add a background image for a specific page:
Create your CSS class for the background you want.
In the header, you can use something like this to check for a specific page.
See Conditional Tags for more ideas of what you can do.
As for the layout of that site, you should contact them and find out if they are using a publicly available theme or if it is a custom design.
You can simply use