.woocommerce ul.products li.product a img {
    filter: grayscale(100%);
    -webkit-filter: grayscale(100%);
    -moz-filter: grayscale(100%);
    -ms-filter: grayscale(100%); 
    -o-filter: grayscale(100%);
    filter: gray; /* Fix per Explorer */
}
.woocommerce ul.products li.product a:hover img {
     filter: grayscale(0%);
     -webkit-filter: grayscale(0%);
    filter: none;
}