Can’t get scaled cover image

Looks very promising, but I can’t seem to get a properly scaled cover image. My basic image is a 3:4 image and I’ve tried several sizes including EXACTY 307 x 407 (which is not 3:4) but I always seem to lose a large part of the lower half of the page. Width generally seems okay. The image crop / image height / image width options don’t actually seem to do much.

Would buy if this worked, because other than that it looks cool!