To load the image, use:

Assets used by the package itself should also be fetched
using the package argument as above.

Bundling of package assets

If the desired asset is specified in the pubspec.yaml
file of the package, it’s bundled automatically with the
application. In particular, assets used by the package
itself must be specified in its pubspec.yaml.

A package can also choose to have assets in its lib/
folder that are not specified in its pubspec.yaml file.
In this case, for those images to be bundled,
the application has to specify which ones to include in its
pubspec.yaml. For instance, a package named fancy_backgrounds
could have the following files:

To include, say, the first image, the pubspec.yaml of the
application should specify it in the assets section:

flutter

assets

packages/fancy_backgrounds/backgrounds/background1.png