How do I keep images proportional on the product grid?

Example site:

On the Products List Setting I have used Thumbnails, Medium (current), and custom to no avail. Thumbs use 150 x 150. Custom I tried 225 x 150, but the images stated at 150 x 150 regardless. I currently have them set to Medium, but the grid is keeping the image height, but shrinking the width instead of keep it proportional.

How do I fix this?