A .jpg file to separate posts.

I am trying to add a .jpg file between every two posts. There are already quite e few threads here about this; I have read and copied code lines as suggested at least in two different 3ds, pasting and modifying lines in my style.css and index.php. But with no result. This is my blog
http://www.stefanome.it/blog/
There must be some fundamentals that I ignore since instruction given by some of you seems to be easy to replicate. Yet they don’t work with me.
The file I would like to repeat once every two post is:
http://www.stefanome.it/blog/wp-content/themes/twentyten/images/line.jpg
Someone could kindly go back to the womb of coding and help me out?
Thanks!