Whatever – Setting up Image Sizes

Please notice: Before you upload images you have to set up image size.

Please go to >Settings >Media and define settings for WordPress media uploads:

Medium size: Max Width 464 – Max Height 9999
Large size: Max Width 732 – Max Height 9999

Whatever Premium Theme image size screen

Some additional info: https://codex.wordpress.org/Settings_Media_Screen

To apply those changes to already uploaded images you can use plugin Regenerate Thumbnails (allows you to regenerate the thumbnails for your image attachments. This is very handy if you’ve changed any of your thumbnail dimensions (via Settings -> Media) after previously uploading images or have changed to a theme with different featured post image dimensions)
https://wordpress.org/plugins/regenerate-thumbnails/

Read More: How to Install Regenerate Thumbnails Plugin