Add a background picture to my posts

Yes it is possible of course. If background picture is static, you can use pure CSS to achieve the goal.

Usually, theme uses CSS class attribute name entry-content for post body. You can add background images to CSS definition of entry-content, for example:

.entry-content {
   background-image: url(images/my-bg-image.jpg);
}

More complex scenario is when you want the background image different for each post. But, you still can accomplish this goal using post-thumbnails. Although it is not easy to explain in one or two sentences.

Hi.
Thanks for the answer.
I couldent get it to work..

I went to
appearace – editor – stylesheet. ( style.css)

then to / * Posts */

and posted .entry-content { background-image: url( "the url link to my picture, which i had uploaded to media); }

But nothing changed.

Where did I go wrong?