Pencil – Setting up Image Sizes

Before you upload images you should set up image size.

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

Best Image Size for Loose Theme:
Medium size: Max Width 494 – Max Height 9999
Large size: Max Width 720 – Max Height 9999

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 image dimensions)
https://wordpress.org/plugins/regenerate-thumbnails/

More info: How To install ‘Regenerate Thumbnails plugin’