Just to add that Russia used Ukrainian prisoners for propaganda already in 2014. In Crimea, then in Donbas.The Geneva Conventions specifically allows one side to ignore most of the conventions IF the other side has already done so. During the meetings that resulted in the GC (and other international conventions) the various parties realized that if one side does not follow the "gentleman's rules" it puts the other side at too much of a disadvantage in many circumstances.
Russia violated the no use of prisoners as propaganda laws within the first 2 months of the start of the war. So did Ukraine - I have no idea who did so first.
There are certain behaviors that are largely considered to be applied regardless of whether a country is party to a particular convention or other international law - knowingly targeting civilians is one of them - but taking pictures of captured prisoners is not one of them. Even torturing combatant prisoners for information is not sacrosanct - as was proven by the US after capturing various members of al-Queda and other organizations listed as extremist/terrorist.
At first, they pretend that prisoners were captured by pro-Russian locals. The cover was blown when they transferred POW Nadiya Savchenko from the Donetsk region to the Russian territory and accused her of unlawful crossing the border. The grotesque show continued in Russian courts for over a year.