This /assets folder is for the WordPress.org SVN "assets" directory — plugin
header/screenshot images shown on the plugin's wordpress.org listing page.
It is NOT shipped inside the installable plugin zip; SVN convention keeps it
as a sibling of /trunk and /tags in the plugin's SVN repository.

Done:

  icon-128x128.png       generated from the brand logo you supplied
  icon-256x256.png       generated from the brand logo you supplied

Still needed before submission:

  banner-772x250.png     772x250, plugin page banner
  banner-1544x500.png    1544x500, retina banner
  screenshot-1.png ...   one per numbered item in readme.txt's
                          == Screenshots == section, matched by index
                          (screenshot-1.png = item 1, etc.) — take these
                          from the live admin UI once it's installed on a
                          real WordPress site.

The banners should reuse the same blue accent (#0A6EE8) and logo mark as
the icons and the admin UI, so the wordpress.org listing matches what
users see after installing.
