Changing Home Page Category Image Thumbnail Size

Hi I'm a new nopCommerce user.

I'm trying to change the thumbnail image size for the categories displayed on the home page. I understand you can do this by going to global settings -> media and changing "Category thumbnail image size:" which has a default size of 125. When I change this to 316 pixels, the images get blown out of the box. Note that I have increased the box size (HomePageCategoryItem in the category.css) to accomodate this.

I have also changed the "HomePageCategoryItem .picture" in category.css to match the box size so that thumbnail images fit perfectly in the box. However they still grow out of the box.

So how does one go about making it so that the category images on the home page fit in the box?

Thanks for any help.