Beluga adds a couple of options that can be modified through WordPress’s Customizer. In the Customizer, you’ll find two added sections: ‘Site License Information,’ and ‘Featured Image Options.’ In the former, you have the ability to modify the copyright notice that appears at the bottom of every page to suit your needs (we, of course, recommend Creative Commons licenses). While in the latter, you have the option to define whether featured images appear on the homepage of the blog, and whether they appear in the headers of single posts. You’ll also find an option to modify the header’s opacity in the ‘Header Image’ section. This allows you to fade your header image a little or a lot, in order to make your title text more visible.
If you require further customization — as with Automattic’s Underscores itself — Beluga theme is built to make it easy to modify. Adding a child theme will give you great flexibility in terms of overriding Beluga’s CSS, adding code for displaying just about any post format in just about any configuration you like, and overriding just about any of the theme’s functions.
If a featured image does not appear in the header of a single post, that is a feature, not a bug. In order for featured images to appear, the image you’ve chosen must be at least 1600px wide. This maintains the integrity of the header’s overall look, and keeps things from getting weird with the white space.