how to add the picture on a background?

good question, I also wondered.

or at least change the background to something else ..

That can be done if you include the image url with an attribute in the body tag of index.php file which can be found on templetes -> default

How to do it? and where to find the Body tag? there is an example?

No, there is not an example..

Can someone write an example of code with an attribute?

Code:

Replace bgimage.jpg with whatever the name of the background file you want to put up.

Code:

where this insert?