Voice theme will automatically generate all image sizes to fit each layout you choose to display. The only thing you should keep in mind is that images shouldn’t be too small to show them in particular areas. That way we can properly generate an image size for each of the layouts.
Basically, with an image that is at least 1140px wide, you can make sure it will cover each of the layouts we provide. Don’t worry about larger images as the theme will resize them properly, just avoid uploading small ones.