To specify that the image be not cropped when shown in a single post/page/portfolio, you can check the “Retain Image Height” option in the blog page in the theme options as shown below –
https://docs.google.com/file/d/0B9j6FQo4cSQFZW11S3lxMkw3Y1E/edit?usp=drivesdk
If you are concerned about cropping in the pages that display multiple portfolio items, the best way is to upload an image which is already cropped appropriately by you in a photo editing tool instead of relying on the system to do it. Changing height of images in these pages will spoil the layout and make the whole arrangement look odd. Hope this helps.