Adding the same background image to all posts & static posts page

Hi,

I have been trying to figure out how to add the same background image to all my posts and to the main, static posts page. Not much luck after weeks of ‘googeling’

At the moment they have the default background (images/bg.jp, just like the rest of the pages on my site. I have uploaded ‘Custom Post Background’ plugin which works perfectly with dynamic but not static pages.
I have found some information about how to change the style.css, page.php and/or header.php in order to customize the background image on the posts page and on the posts, but found them very confusing. Is there any way I can work out some coding? Attention! I am a newbie to WordPress!

I would appreciate any help. Thank you!