As of this post, Quark is the default theme for Grav websites. It has a number of configurable options that you may want to check out.
As mentioned in the tutorial, there are two ways to add images to your pages. The tutorial walks you through uploading an image and then setting it as the "hero image" used by the page template. It also provides a brief description of adding images within the content. In this post we will demonstr...
To quote the Grav documentation:
Grav is a Fast, Simple, and Flexible file-based Web-platform.