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 on particular areas. That way we can properly generate an image size for each of the layouts.
Basically, with image that is at least 1140px wide you can make sure it will cover each of layouts we provide. Don’t worry about larger images as theme will resize them properly, just avoid to upload small ones.