Mai usato jquery, se ti puo essere utile lo puoi fare tranquillamente in vanilla javascript ES6.



codice:
 const gallery = document.querySelectorAll('.product-gallery-view > img')
        let counter = 0

        function cssClass() {
            gallery.forEach((img, idx) => {
                counter = idx + 1

                counter < 10
                    ? img.classList.add(`xx_0${counter}`)
                    : img.classList.add(`xx_${counter}`)
            })
        }
        window.addEventListener('DOMContentLoaded', cssClass)