Which Dimensions for the Preview Thumbnails?

Hi HerbyDE. When you upload an image to the Media Library, WordPress creates the following thumbnails:
‘thumbnail’ 150px x 150px
‘medium’ 300px x 300px
‘large’ 640px x 640px
‘full’ original size uploaded

In addition, the theme also creates the following cropped thumbnails:
‘thumb-small’ 160px x 160px
‘thumb-standard’ 320px x 320px
‘thumb-medium’ 520px x 245px
‘thumb-large 720px x 340px

You can read more about the theme thumbnails in the documentation.

The theme uses the following images:
Featured post: thumb-large (scaled proportionately to fit the content width depending on the sidebar layout)
Font page normal blog layout: thumb-medium
Front page Standard Blog Layout: thumb-standard
AlxPosts widget: thumb-medium
AlxTabs widget: thumb-small
Related posts: thumb-medium

If you’re interested here are several references regarding image sizing and cropping:
http://havecamerawilltravel.com/photographer/wordpress-thumbnail-crop
http://havecamerawilltravel.com/photographer/wordpress-resize-thumbnails
http://www.davidtan.org/wordpress-hard-crop-vs-soft-crop-difference-comparison-example/
http://www.wpexplorer.com/wordpress-image-crop-sizes/

Hope that helps.