Hi,
I checked your link but I don't see a problem with the images popup.
It's only the main image thumbnail which is distorted there.
And that's because of this CSS:
monosnap.com/file/80ETzphw0VjbzkEyXYvJT6HdkJmtn0
Now I don't know where that CSS comes from as your CSS is compiled by your template or another extension, which hides the origin of the CSS code.
What you can do though is to add such CSS on your website:
img#hikashop_main_image {
height: auto !important;
}
It will override the CSS you already have there and prevent the distortion from occurring on small resolutions.