Okay, now I know that, but where can I those what picture will be in the boxes on this link: https://wordpress.com/theme/cubic (to the top right there are six boxes). If I get the feature image fitting, this one looks very weird ( with black bars top and bottom)
How do I make sure that when I get in a post, the big banner image (is that what you call a feature image) is fitting, where can I chose what picture will be in there?
