Images
Images from this folder are managed by webpack, and can be imported like this:
import mediaPostImageURL from 'calypso/assets/images/upgrades/media-post.svg';
You can then use such image that way:
<img src={ mediaPostImageURL } alt="Illustration" />;
Technically speaking, webpack will copy that image to the public/images output folder automatically, with an immutable hashed name (so this name will change if the source image changes).