Structure uses that big gray area to display a “Featured Image” from your latest post ie. from a featured category of your choice. To change the featured category, go into Appearance > Theme Options.
Small: 440 x 240 on the home page for featured posts
Medium: 620 x 380 on the home page for latest feature post
Large: 640 x 392 on single posts
A featured image will not display if it’s smaller than the dimensions required by the theme.