nav-wrapper {
background-image: url('../cluj.jpg');
height: 300px;
width: 100%;
}

My first question, if we were pair-programming would be "Where is your image located in relation to this page?". For example, if this page is /mySite/assets/css/myStyles.css and the image you're referencing is in /mySite/assets/img/cluj.jpg, then you're pointer would need to be corrected since it would be pointing to /mySite/assets/.

If you're following along with the video, your images are in a folder called img and the CSS is in a folder called css if I'm not mistaking. If that's the case, then you need to:

background-image: url('../img/cluj.jpg');

This tells the browser to come out of the css folder, by 1 directory, and look for a folder called "img", then inside that folder, look for the image called "cluj".

Hopefully this helps!

EDIT Here's a bit more graphical explanation of traversing file structures.

Home location: //webroot/index.html
CSS location:  //webroot/css/myStyles.css
Image assets: // webroot/img/myImg.jpg

From within the CSS:
background-image: url('../img/myImg.jpg');

This tells the browser that, starting from //webroot/css/myStyles.css, go back by 1 directory which will bring it to //webroot/, then look for a folder called img and go into that directory. This will bring us to //webroot/img/, and finally look for the file myImg.jpg.