Yandex N.V. is a multinational corporation primarily for Russian and Russian-language users, providing internet services, such as Yandex Drive. Piktures is NOT associated with Yandex.
Piktures is able to manage, view, download from, and upload photos to Yandex Drive.
Piktures uses Box WebDav access to execute those operations. As a Piktures user, you will have to enter your username and password. The credentials will be stored locally on your smartphone. They are not sent anywhere outside of this device.