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)