You would be better off creating a child theme, this will ensure your edits do not get wiped out when you update Woo.
Anyway, this CSS should work. I usually avoid using !important but without seeing your page it’s hard to say what is overriding your styles.
.woocommerce #content div.product div.images,
.woocommerce div.product div.images,
.woocommerce-page #content div.product div.images,
.woocommerce-page div.product div.images{
width:40% !important;
}