You can't change the CSS to fix this because the image is being cropped at the point of upload. Ideally, you'd need to upload a uniform image that has the suggested dimensions of 837x710 pixels but that's a little arbitrary and not really the best solution if you've already got a standard uniform size of your own.
I'll need to look in to reworking the uniform portion of the add-on to better allow a wider range of uniform sizes.