Generating thumbnails for posts

@Michael Bisset:

I´m working with the gridster theme, taken from the https://premium.wpmudev.org/blog/30-free-responsive-and-stunning-wordpress-themes/ which you recommended.

I have figured out how to categorize and publish posts, but I can´t figure out how to generate thumbnails for the posts.

Like in; http://themefurnace.com/selector/?theme=Gridster where all the posts have beautiful thumbnails.

Is there a plugin for this, or do I need to learn CSS?