Yes, the first image has the srcset’s:
<a href="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044.jpg" itemprop="image" class="woocommerce-main-image zoom" data-thumbnail-src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-260x330.jpg" title="136S1044" data-rel="zoomImage"><img width="510" height="652" src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-510x652.jpg" class="attachment-shop_single size-shop_single wp-post-image" alt="Green georgette Saree" data-thumbnail-src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-260x330.jpg" title="136S1044" srcset="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-510x652.jpg 510w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-235x300.jpg 235w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-768x982.jpg 768w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-801x1024.jpg 801w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-600x767.jpg 600w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-720x921.jpg 720w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044-117x150.jpg 117w, https://www.shopzepp.com/wp-content/uploads/2016/05/136S1044.jpg 860w" sizes="(max-width: 510px) 100vw, 510px" /></a>
and the second doesn’t:
<a href="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1042.jpg" itemprop="image" class="woocommerce-main-image zoom" data-thumbnail-src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1042-260x330.jpg" title="136S1042" data-rel="zoomImage"><img width="510" height="652" src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1042-510x652.jpg" class="attachment-shop_single size-shop_single wp-post-image" alt="Georgette White orange saree" data-thumbnail-src="https://www.shopzepp.com/wp-content/uploads/2016/05/136S1042-260x330.jpg" title="136S1042" /></a>
If you find out why please also let me know. I didn’t have time to look into the installed plugins you have on the website.