so that the image is seen correctly in all resolutions you should replace your code:
body{
background-image: url("static/img/joblist.jpg");
height: 100vh;
}
for this:
body {
background: url("static/img/joblist.jpg") no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}
This is what covers the entire body of the page depending on how large the browser window is and also compatible with all browsers.
Also as Christian said, make sure that the "link" is inside the "head" header and that the "static / img / joblist.jpg" path is correct.
For the first, you would have to locate the master page or template, since "portada.html" is a simple "div" so there it will never work for you no matter how much you put the "link" to your CSS, you must have a file that contains the template of the page (master page) or header, I recommend you look for the tag "head" between the files of your project with the search engine of your editor.
Anything you tell us.