Custom size for photos inside article

I already know how to define a custom size in functions.php

add_image_size( ...

But I dont know how to automatically setup wordpress to put the thumb style in all pictures added via the post editor.

For example here

http://peiraxtiri.3media.gr/envato-network-one-big-monster-is-coming/

you will see a picture in the middle that is actually huge. What I want is to display the thumb version inside the article and when it is clicked to display the full version.

Please guide me :slight_smile: