Tested on Django 2.1 and Python 3.5.
Example settings:
OPPOSABLE_THUMBS = {
'CACHE_DIR': os.path.join(BASE_DIR, 'path/to/folder/'),
'ALLOWED_SOURCES': [
'http://example.com/static/one/',
'http://example.com/static/two/',
'http://somwehereelse.com/',
]
}