How to add a gradient on the image of the Stellar layout?

Copy the below snippet to your CSS file or Divi Theme Options Custom CSS area

.el-single-post-template-stellar .el-featured-image:before {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    content: '';
    background-image: linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0));
}