Which Whiteboard service to choose for a psychological experiment?

Question

Work organization

Hello!
Please tell me a good whiteboard service for organizing remote testing of subjects using methods that involve the use of cards.
The essence of the task is as follows: there are two (always exactly two) people - the experimenter and the subject. They are at a distance from each other and have contact by phone or skype.
The experimenter must provide the subject with a certain set of cards (pictures) in the amount of 10 to 64 pieces. And the subject, following the instructions of the experimenter, must arrange these cards into groups.
Actually, the task is to organize the process so that the subject does not need physical cards.
In general terms, I see the solution to this problem as follows: you need to find some kind of whiteboard service that will allow you to organize this. This service should have the following qualities:
1. Possibility to upload your images there (photos of cards);
2. The ability to move these images around the virtual board space. Both participants in the process, both the experimenter and the subject, must have this opportunity;
3. The ability to overlay images on top of each other (like cards in a stack);
4. Online display of the process of moving cards for both participants in the process (i.e. if the subject moved the card, the experimenter should see it immediately, without the need to refresh the page or perform other additional actions);
5. Free or low price. Any business features are not needed, the functions described here are enough.
6. It will be generally great (but not necessary) if it is possible to create and save for later use several different boards with different sets of cards.
7. Ideally, this should be a WEB service that does not require the installation of any application, plug-ins, modules, etc.
I know that there are quite a lot of similar services, but so far I have not been able to find one among them that would fit in all respects.