Apply Modules to Previous Posts
For colorbox to work on your new posts, you need to edit the content type(s) of your new posts

Home » Administration » Structure » Content types » YourContentType


Click on "Manage Display". Here, you can choose the format "Colorbox" for the image field (and click that modal to customize, etc). It will be applied to all nodes of that content type. 

Of course, you'll first have to install the colorbox module and library. 

The same principle applies to other modules. 

About posts/nodes and content types:

Typically, there are only very few content types configured in Drupal - by default there are "article" and "basic page", but you can create more as you need them. 

You would import your old post into drupal using one or very few different content types.

The changes need to be done for the content type, meaning they'll get applied to all nodes (posts) in that content type. 

You need to look which content type to imported your nodes (posts) into. When you go to http://www.example.com/admin/content, you see a list of your nodes, and in the "Type" column the content type. 

Then you go to http://example.com/admin/structure/types/manage/yourcontenttype/display and manage the display as outlined in my answer. 

For the article content type, this would be:
http://example.com/admin/structure/types/manage/article/display

Of course, you need to change example.com to your url.