flutter

assets

directory/

directory/subdirectory/

Note:
  Only files located directly in the directory are included unless
  there are files with the same name inside a subdirectory 
  (see Asset Variants). To add files located in subdirectories, create 
  an entry per directory.

Asset bundling

The assets subsection of the flutter section
specifies files that should be included with the app.
Each asset is identified by an explicit path
(relative to the pubspec.yaml file) where the asset
file is located. The order in which the assets are
declared doesn’t matter. The actual directory name used
(assets in first example or directory in the above
example) doesn’t matter.

During a build, Flutter places assets into a special
archive called the asset bundle that apps read
from at runtime.

Asset variants