As mentioned on the related issue about UserService::loadUserGroup we can load any kind of content.
Here also, we can directly assign any kind of content objects to the created user

Steps

1. go to the admin interface
2. create a folder and get it's object ID (<folder-id>)
3. create an image and get it's object ID (<image-id>)
4. load the content objects using public API

       $userCreateStruct = // make a valid user create struct
       $parentGroups = array();
       $parentGroups[] = $contentService->loadContent( <folder-id> );
       $parentGroups[] = $contentService->loadContent( <image-id> );
       $userService->createUser( $userCreateStruct, $parentGroups );
   

5. Verify no exception was thrown
6. Verify that on the created folder the user is shown
7. Verify that on the created image (non-container) the user is not shown