Hi,
Ok, I see what you mean. What you're talking about is when the image opens as a new page. Here, it opens inside a popup. So it's normal you can't zoom in as popups don't allow for that.
If you want to avoid the stretching of the image, you can add such CSS on your website:
.sbox-content-image img, #sbox-window {
height: auto !important;
}
Otherwise, one thing you can try is to change the "image popup mode" option of the Display tab of the HikaShop configuration. It might give you a better display as it will then use another image popup system than the default one