Featured Image – Front page Thumbnail and Page image sizes.

Morning All

Hopefully a simple one, please bare with me as I am completely new to WP after recently switching from Drupal. I’ve had a look around for existing questions but have had no luck so far.

New posts are featured on the index home page as well as the top of the post.

The index image displays at 300×220. Class = attachment-post-thumbnail wp-post-image.
The post page’s image displays at 620×220. Class = featured-thumbnail large.

My question is how do I set the image to display unaltered on the home/index page, but be cropped to show the desired part of the image on the post? I imagine i’m simply not using the featured image tool correctly.

Thanks in advance.

Stephen