I advise not to add the code to style.css. This file will be overwritten at next theme update.
In the the Virtue theme, custom css is added at Admin page > Appearance > Theme Options > Advanced Settings.
You don’t need JetPack if you just need the custom css function.
There are different ways of doing images. Personally I don’t like the hard crop feature because sometimes you get parts of the image cropped that you don’t want cropped. I don’t like forcing a set size because some images get stretched or squashed and don’t look right. For best results, I would standardise on an image aspect ratio for the site, and hand-edit all images to be that aspect ratio in an image editor before uploading. If all your images have the same aspect ratio they will show in neat rows.
The catalogue image size set at Admin page > WooCommerce > Settings > Products tab > Products Image sizes, should be about the same size that they are used on the page, 270px in your page. If the setting size is set too small, the thumbnails are enlarged to fit the space set by the theme and can look a little pixelated.