Broadcast theme size of the product images

[Kris34]

Hi there

I'm using the Broadcast theme and I would like to change the allowed size of the product images on the product page from 1:1 to 3:4.

By default, the image ratio is 1:1. When I upload images that are 3:4, it creates white (blank) spaces on the left and on the right side of the image. 

Could anyone give advice on how to edit the size resp. the ratio of the product images?

Best

Kristina