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: http://prntscr.com/ag6zqx

This is the test site: http://emprendedor.staging.wpengine.com/temas/desarrollo-personal/ http://prntscr.com/agyhmx

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 https://developer.wordpress.org/reference/functions/add_image_size/ but no luck there.

These are our php's:
- Category.php http://prntscr.com/agxfe2
- Functions.php http://prnt.sc/agxg49

Any ideas?
Thanks!