You may set up image size in the Settings>Media section. Try adjusting the Landscape size (Large) field. Also, please, make sure that the pictures’ size is at least 400x300, then when uploading, they’ll be cropped to correspond to the set proportions.
Please, try uploading pictures not smaller than 400x533; this will allow proper cropping to suit image proportions. If it doesn’t help, you can send us the link to your website, and we’ll check the issue.