Automatic resizing of images / thumbs
The theme uses, like almost any theme, custom image sizes that are set by the theme. The sizes used on the homepage depend on what layouts you use when setting up the homepage with the homepage builder.
The images are always sized and cropped if needed automatically by Wordpress. In order to make it work right, you need to use images which are not too small, and ( depending on the layout you build ) with about the same ration so the cropping will be mimimal.
Also note that after activating the theme, only newly uploaded images are sized and cropped with the right settings. If you already had used uploaded images before installing the theme, it can’t find the right sizes because. In that case you need to either re-upload the images, or use a plugin like force-regenerate thumbnails.