スライダー用の画像のCSSが適用される前に表示されてしまう

困っていること
Swiper.jsを利用しサイトのメインビジュアル部分をスライドさせようとしています。

最終的な表示は下記のスクリーンショットです。↓

ただ、ページをロードすると、読み込み途中にCSS適用前の画像が横並びに表示されてしまいます。↓

ユーザーには常に1枚目の状態で画像を表示させたいのですが、画像2枚目のような表示を防ぐ方法があればご教示いただけますと幸いです。

よろしくお願いいたします。

追記

コードの提示をご依頼いただいたので記述いたします。

<div class="main-visual">
    <div class="add-wrapper">
        <div class="swiper-container swiper1">
            <div class="swiper-wrapper">
                <div class="swiper-slide slide1">
                    <a href="<?php echo MODULE_WEB_ROOT; ?>findinfo" class="cover"></a>
                </div>
                <div class="swiper-slide slide2">
                    <a href="<?php echo MODULE_WEB_ROOT; ?>findinfo" class="cover"></a>
                </div>
                <div class="swiper-slide slide3">
                    <a href="<?php echo MODULE_WEB_ROOT; ?>findinfo" class="cover"></a>
                </div>
                <div class="swiper-slide slide4">
                    <a href="<?php echo MODULE_WEB_ROOT; ?>findinfo" class="cover"></a>
                </div>
                <div class="swiper-slide slide5">
                    <a href="<?php echo MODULE_WEB_ROOT; ?>findinfo" class="cover"></a>
                </div>
            </div>
            <!-- swiperのページネーション -->
            <div class="swiper-pagination page1"></div>
            <!-- ナビゲーション -->
            <div class="swiper-button-prev prev1"></div>
            <div class="swiper-button-next next1"></div>
        </div>
    </div>
</div>

.main-visual {
  width: 100%;
  height: 522px;
  background-image: url(../background-image);
  background-repeat: repeat;
  z-index: 0;
  background-color: #F0F0F0;
  overflow-x: hidden;
  overflow-y: hidden;
  position: relative;
}

.add-wrapper {
  margin-left: auto;
  margin-right: auto;
  max-width: 900px;
}

.swiper1 {
  width: 100%;
  overflow: visible;
  z-index: 1;
  height: 520px;
}

.swiper1 .swiper-slide {
  vertical-align: middle;
  margin-top: 88px;
  position: relative;
  transition: all .7s cubic-bezier(0.165, 0.84, 0.44, 1) 0s;
  height: 335px !important;
}

.swiper1 .swiper-slide .mv-img {
  width: 670px !important;
  height: 335px !important;
}

.swiper1 .swiper-slide-active {
  height: 450px !important;
  width: 900px !important;
  margin-top: 30px;
}

.swiper1 .swiper-slide-active .mv-img {
  height: 450px !important;
  width: 900px !important;
}

.swiper1 .swiper-slide-prev,
.swiper1 .swiper-slide-next {
  width: 100% !important;
  height: 335px !important;
}

.swiper1 .swiper-slide-prev::after,
.swiper1 .swiper-slide-next::after {
  content: "";
  position: absolute;
  display: block;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.7);
}

.swiper1 .swiper-slide-prev .mv-img {
  width: 670px !important;
  height: 335px !important;
  position: absolute;
  top: 0;
  right: 0;
}

.swiper1 .swiper-slide-next .mv-img {
  width: 670px !important;
  height: 335px !important;
  position: absolute;
  top: 0;
  left: 0;
}

.swiper1 .swiper-pagination {
  width: 100%;
  left: 0;
  bottom: 10px;
}

.swiper1 .swiper-button-next,
.swiper1 .swiper-button-prev {
  transform: translateY(-50%) translateX(-50%);
  -webkit-transform: translateY(-50%) translateX(-50%);
  margin-top: 0;
}

.swiper1 .swiper-button-next{
  right: calc(-50vw + 470px);
  width: 32px;
  height: 60px;
  background-image: url(../swiper-button-image);
}

.swiper1 .swiper-button-prev{
  left: calc(-50vw + 496px);
  width: 32px;
  height: 60px;
  background-image: url(../swiper-button-image);
}

.swiper-pagination-bullet-active {
  opacity: 1;
  background: #91d539;
}

.swiper1 .slide1 {
  background-image: url(../slide-image);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}
.swiper1 .slide2 {
  background-image: url(../slide-image);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}
.swiper1 .slide3 {
  background-image: url(../slide-image);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}
.swiper1 .slide4 {
  background-image: url(../slide-image);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}
.swiper1 .slide5 {
  background-image: url(../slide-image);
  background-position: center;
  background-size: cover;
  background-repeat: no-repeat;
}

window.addEventListener('load', function() {
    var mySwiper = new Swiper ('.swiper1', {
    loop: true,
    slidesPerView: '1',
    centeredSlides : true,
    loopedSlides:5,
    spaceBetween: 0,
    clickable: true,
    breakpoints: {
        960: {
            slidesPerView: 1,
            spaceBetween: 0
        }
    },
    navigation: {
        nextEl: '.next1',
        prevEl: '.prev1',
    },
    pagination: {
        el: '.page1',
    },
    autoplay: {
      delay: 7000,
      disableOnInteraction: false
    },
    speed: 700,
    roundLengths: true,
    });
}, false);