How do I add a post divider to K2 theme?

This will add a line:

.post{
    border-bottom: 1px solid #000;
}

This will add an image:

.post{
    background: transparent url('YOUR URL HERE') no-repeat 50% 100%;
}

You may need to adjust some parameters:
http://www.w3schools.com/css/css_background.asp

YES in the style.css of your current theme

SEarch for .post and it there