swiper 图片滑动

swiper滑动，图片轮播时，图片已设置圆角20rpx，但是开始轮播的时候，每一个图片滑动，都会变成直角，应该怎么做？

    <!-- 轮播图 -->  
    <view class="swiper-box">  
        <swiper class="swiper" circular autoplay interval="3000" indicator-dots indicator-color="rgba(255, 255, 255, 0.5)" indicator-active-color="#fff">  
            <swiper-item class="swiper-item">  
                <image class="img" src="/static/主板.jpg" mode="aspectFill"></image>  
            </swiper-item>  
            <swiper-item class="swiper-item">  
                <image class="img" src="/static/内存.jpg" mode="aspectFill"></image>  
            </swiper-item>  
            <swiper-item class="swiper-item">  
                <image class="img" src="/static/显卡.jpg" mode="aspectFill"></image>  
            </swiper-item>  
        </swiper>  
    </view>  

// 轮播图  
.swiper-box{  
    width: 100%;  
    padding: 30rpx;  
    .swiper{  
        width: 100%;  
        height: 350rpx;  
        .swiper-item{  
            width: 100%;  
            height: 100%;  
            border-radius: 20rpx;  
            overflow: hidden;  
            .img{  
                width: 100%;  

                height: 100%;  
                border-radius: 20rpx;  
            }  
        }  
    }  
}