Product item second image is transparent or smaller

If you enable the feature "Show second image on hover", but your second image is transparent or smaller than the first image. You will see the first image behind the second image. You can fix it by add the below CSS to your Footer Script (Manager panel > Storefront > Footer Script)

<style>
.card-image + .card-image {
    background: #ffffff;
    width: 100%;
    height: 100%;
    object-fit: contain;
}
</style>