Ok, I solved it by adding the url for the background image at the end of the line.
Here’s my working code:
.css {
background:
linear-gradient(to bottom, rgba(0, 0, 0, 0) 0%, rgba(0, 0, 0, 0) 59%, rgba(0, 0, 0, 0.65) 100%),
url('https://cdn.sstatic.net/Sites/stackoverflow/company/img/logos/so/so-icon.png?v=c78bd457575a') no-repeat;
height: 200px;
}<div class="css"></div>