Image boxing with whitespace

I’ve created a custom image set with different image formats, all of them are in the 2:3 aspect ratio. If we upload an image with a portrait format the gallery processor creates an image that fits in the height but not in the width and the image isn’t boxed with white space. Cropping and scalling are disabled in the settings.
I think the bloomreach experience manager will box the image automatically, if the original doesn’t fits into the image variant.
How can I become boxed image variants?