field type display_settings file upload field?

For a website I am designing, I am developing a custom field that requires an image is displayed in the field when the content is entered. When the field is added to my custom fields group, I’d like the admin to be able to upload this image.

I know I can use

$file_field = $this->EE->file_field->field(
   $this->field_name,
   $data,
   $allowed_file_dirs,
   $content_type
  );

in the display_field() function inside my custom field. Is there a way it can be included in the display_settings() function?

Also, I’d like to inject some javascript on the entry page using:

$this->EE->cp->load_package_js('my_file');

However what directory do I place my javascript in?