Taxonomy archive image sizes-not resizing proportionally

Hi there!
We're stuck with the taxonomy image sizes, as not all of them show in same size:

This is the test site:

We've tried a css:

body.archive img.attachment-.size-.wp-post-image {
    width: 268px !important;
    height: 179px !important;

However this method resizes, but does not respect proportions. We've checked out but no luck there.

These are our php's:
- Category.php
- Functions.php

Any ideas?