How do I add a post divider to K2 theme?

There are thousands of WordPress themes – which means that many people won’t be familiar with your particular theme. So a link to your site where people can see the theme might result in more responses.

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:
https://www.w3schools.com/css/css_background.asp

Dear all

were should i put the above code, is it style.css??

YES in the style.css of your current theme

SEarch for .post and it there