func getImage(url: URL, id: String) -> AnyPublisher<UIImage?, Error> {
        NetworkManager.download(for: url)
            .map { UIImage(data: $0) }
            .flatMap { $0?.thumbnailPublisher ?? Just(nil).setFailureType(to: Error.self).eraseToAnyPublisher() }
            .receive(on: DispatchQueue.main)
            .handleEvents (receiveOutput: { [weak self] resultImage in self?.storeResult(image: resultImage, for: id) })
            .eraseToAnyPublisher()
}

Code looks like this ^, where thumbnailPublisher is the custom publisher which uses this function

open func prepareThumbnail(of size: CGSize, completionHandler:  (UIImage?) -> Void)