using image in stylesheet

thanks @andrew
The theme has a slider for posts of the blog, the only place I can find the codes for this slider is in style.css and all of the codes for this slider is:

&.mpc-vc-blog-posts-slider {
			.mpcth-slide-wrap {
				float: left; // non-RTL
			}
			.mpcth-slide {
				position: relative;
				display: block;
				overflow: hidden;
				padding-right: 20px;
				padding-left: 20px;
				height: 152px;
				float: none;
				max-width: none;

				&.mpcth-slide-row-gap {
					margin-bottom: 1.5em;
				}

				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;
				}

				.mpcth-slide-title {
					text-transform: uppercase;
					margin: 1em 0 .25em;
				}
				.mpcth-slide-time {
					text-transform: uppercase;
					color: #666666;
					font-size: .923em;
				}
				.mpcth-slide-text {
					color: #666666;
					text-align: justify;
					line-height: 1.5em;
				}
				.mpcth-slide-trim {
					position: absolute;
					bottom: 0;
					left: 0; // non-RTL
					background: linear-gradient(to bottom, rgba(255,255,255, 0) 0%, rgba(255,255,255, 1) 90%);
					background-image: -webkit-gradient(linear, 0% 0%, 0% 90%, from(rgba(255,255,255, 0)), to(rgba(255,255,255, 1)));
					background-image: -moz-linear-gradient(rgba(255, 255, 255, 0) 0%, rgba(255, 255, 255, 1) 90%);
					filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#00ffffff, endColorstr=#ffffffff);
					-ms-filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#00ffffff, endColorstr=#ffffffff);
					width: 100%;
					height: 2em;
					z-index: 1;
				}

				@media #{$screen-large} {
					.mpcth-responsive & {
						height: 173px;

						img {
							width: 260px;
						}
					}
				}
				@media only screen and (max-width: 640px) {
					.mpcth-responsive & {
						height: auto;
						padding-left: 0;
						padding-right: 0;

						img {
							float: none;
							width: 100%;
							margin: 0 0 1em 0;
						}

						.mpcth-slide-wrapper {
							padding-left: 20px;
							padding-right: 20px;
						}
					}
				}
			}
		}

and the codes for the image I think is the codes in my previous post. This code calls a image with 200px width auto height but actually it loads a 600×450 image wich defined in functions.php as “horizontal-2” and in GTMETRIX my website has a “Serve scaled images” problem wich loads more than 150KB for this difference in image loading. Can I call the predefined “hoizontal-4” image in functions.php instead of the image style.css calls as 200px width?
a code like:
<?php the_post_thumbnail( $size, $attr ); ?>
in php files.