using image in stylesheet

In my theme there is a code for blog posts slider, in a part of that is this code for thumbnail images that show in this slider:

img {
                    position: relative;
                    z-index: 2;
                    display: block;
                    width: 200px;
                    height: auto;
                    @include float(left); // float: left;
                    @include margin-right(20px); // margin-right: 20px;
                    @include margin-left(-20px); // margin-left: -20px;
                }

the problem is where it calls a 200px image in width but I wanna call the image that defined in functions.php with the name "horizontal-4". how can I call that?