Problem with image overflow just in some articles

Your CSS:
div.hikashop_product_main_image_subdiv a img#hikashop_main_image {
max-height: 300px;
width: auto;
}

should actually be:
div.hikashop_product_main_image_subdiv a img {
max-height: 300px;
width: auto;
}

And that way you wouldn't have that issue.

Add also that CSS:
.hikashop_product_main_image{ height: auto !important;}

It basically happens because you didn't force the height on the product page image in the configuration but only the width and thus the system takes the height of the first image while on that seconde page the height of the other images is bigger than for the main one and that's why you get the overlap. That CSS will allow the div height to adapt to the height of the current image and avoid the overlap.