How do I put a background image on website

Hello,

Am trying to place a background image on a T3 framework (latest version) website.

Have tried using the 'body' tag in a custom.css file - didn't work.

Have tried using the Javascript plugin called 'backstretch' - couldn't get it to work.

Have tried to figure out how to create a class that would be loaded in the 'body' tag in the hope that might work - can't figure it out.

Was hoping this would be a simple thing to do but can't seem to do it.

Can you please give a detailed explanation of how to do it, I'm sure many people would be interested.

Kind regards,
robjen