I would like to have a background image on my site, but only on the sides of center column and sidebar, how can I do this?

/K

the background image is applied to the entire page. However you can then style the other areas to allow it to show thru or not.
you have a header area
next comes the sidebars and center area
then comes the footer
each of these areas can be further broken down, for example, the center area is comprised of the posts and the post box can be styled.

I suggest you put in a background then in every area you can style in the theme, set a different background color so you can see all the areas.

Thanks, worked just fine!