Custom thumbnail source

Sorry for late reply.

The feature you are looking for only available in premium version

Please see the screenshot bellow, you can use “Custom Thumbnail” on media source and add your image via post grid meta options for each post.

http://i.imgur.com/OYskywe.png
http://i.imgur.com/ziejTSu.png

Let me know if you are interested in premium version.
you may take a look here
http://pickplugins.com/item/post-grid-create-awesome-grid-from-any-post-type-for-wordpress/

Regards