We've just packaged v0.1.2 of the Baseline theme, which adds support for featured images (here's an example). The installation instructions are here.

Our next goal is to get the theme accepted into the WordPress.org theme directory. That will make it a lot easier to install on self-hosted sites, and and is a prerequisite to getting the theme approved for WordPress.com. 

While not strictly necessary for acceptance, in that process we'll be converting Baseline to a block theme. We want to be certain Baseline is compatible with future versions of WordPress, and a block theme will help ensure that.